The Goa'uld are a parasitic race having the appearance similar to small snakes. They are capable of infesting other beings, such as humans, and taking control over them. they first reached out of their homeworld when they infested a neighboring species, the Unas. With control of the Unas, they went into exploration through the Stargate and found many artifacts of Ancient technology, which allowed them to forge an empire across many planets. When they stumbled upon the primitive humanity, they discovered that they make excellent hosts, and enslaved them. Seeding the human throughout the galaxy, they managed to use their new found slaves, hosts and warriors, to gain control over the entire Milky Way Galaxy.
Centuries of being worshiped has given the Goa'uld a massive God complex. They actually believe themselves to be divine (Goa'uld means children of the Gods in their language) and they use their technology to convince their slaves of that as well.
Though the Goa'uld themselves are massively technologically advanced, the humans which they rule are not. In fact they are generally stuck in a middle ages level of development.
The Goa'uld evolved on a swampy world along with a humanoid-reptilian race called the Unas (the first one, as in the first host race). The Goa'uld physiology allowed them to take Unas as hosts and control their bodies. Soon they learned to control the Stargate left on that planet and started to spread across the galaxy in search of other artifacts of the Ancients. The first leader of the Goa'uld was Atok, but in 22,000 BC, he was murdered by his son Apep, who took control of the Goa'uld. Apep divided his power among minor Goa'uld, among which were Ra. However, in 17,800 BC, Apep was murdered by a Goa'uld very close to him: Anubis.
The devious Anubis declared himself emperor of all Goa'uld, but the lords Apep put in power would not stand for that, and under leadership of Ra, the Goa'uld threw down Anubis and exiled him from their presence.
Ra then declared himself the Supreme System Lord of the Goa'uld Empire. He was a de facto emperor, but maintained the quasi-feudal order Apep had created. This made the other Goa'uld peaceful, and of course, having the largest armies, no one would dare Challenge Ra for supremacy. For the next 18,000 years, Ra would reign supreme over Goa'uld.
Later on, the Goa'uld made contact with the Asgard. Old allies of the Ancients, the Asgard saw the Goa'uld as a threat to the Galaxy. the Goa'uld attacked the Asgard, but soon found out that the Asgard technology was easily more than a match from whatever they reverse engineered from the Ancients. For the next few thousand years, the Goa'uld were on the defensive, as the Asgard were seemingly indestructible, and their homeworld in another galaxy could not be reached by the Goa'uld.
Things were about to change, however, when Ra stumbled upon Earth, and on Earth made contact with the primitive humanity. Ra transferred himself into a human, and found that they were far better hosts than the Unas, being easily reparable, and with Goa'uld technology, sustained indefinitely. He brought a Stargate to Earth (the original one was buried under Antarctica) and him and other System lords began to take humans as slaves. They would later be remembered as Gods of many pantheons on Earth.
The Asgard, having went to war with the replicators in their own galaxy, were forced to cease their war with the Goa'uld. Not wanting to leave humanity completely in the hands of the Goa'uld, they arranged a protected planets treaty with the Goa'uld. The Asgard seeded humans on several planets throughout the galaxy, and deemed them protected planets. Should any Goa'uld step foot on these planets, an Asgard attack on the System lords would soon follow. It was later revealed that this treaty was a bluff, as the Asgard didn't have the manpower to wage war with both Goa'uld and the Replicators, but the Goa'uld, unaware of the replicators, respected this treaty for thousands of years out of fear of the Asgard. The Goa'uld later used the Humans as slaves, and genetically engineered some of them to become Jaffa, the warriors of the Goa'uld.
Modern history - War with the Tau'ri
The Goa'uld eventually suffered a rebellion on Earth and were forced off it. But apart from rebellions of their slaves, since the end of war with the Asgard, they ruled unchallenged in Milky Way for thousands of years, until 1994, when Humans of Earth rediscovered and learned how to activate their Stargate. A team consisting, among others, of Dr. Daniel Jackson and Col. Jonathan "Jack" O'Neill, were sent to the planet Abydos at the same time as Lord Ra came to the planet. Their mission culminated when Col. O'Neill transported a nuclear bomb on Ra's ship, killing him. The Tau'ri (as they would later be known) were not heard from for three years. The contact with the Tau'ri was the beginning of the End for the Goa'uld.
Death of the Supreme System Lord left a power vacuum among the other system lords which was filled by Apophis, brother and greatest enemy of Ra. Apophis, in search of a host for his queen, came to Earth (not realizing where he was), and forced the Tau'ri to restart their Stargate program. The Tau'ri, and especially the premier team known as SG-1, proved to be a match for the Goa'uld, who had not fought anyone but themselves for ages. They were first recognized as a threat by Apophis when they convinced his first Prime (the Jaffa in charge of his armies), Teal'c to defect and free the slaves from the planet Chulak. Over the next years, the Tau'ri were able to inflict defeats on the Goa'uld through their guerilla, but were unable to face them on open battlefield. In 1998, Apophis used two of his greatest motherships to launch an attack on Earth, which was thwarted when SG-1 and Bra'tac, an elderly Jaffa master who came to realize the Goa'uld are not gods, sabotaged his ships.
Apophis lost a considerable standing among other Goa'uld due to that defeat, and eventually fell prey to another Goa'uld, Sokar (who inspired and took the personality of the Devil while on Earth). Soon, Tau'ri inflicted another defeat on the Goa'uld when they killed Hathor, one of Ra's queens. This prompted the System Lords to launch another attack on Earth, but were unable as the Tau'ri made an alliance with the Asgard, and, in 2000, fell under their protected Planets treaty. This treaty kept the Goa'uld from attacking Earth directly, and the Tau'ri were still able to attack the System Lords through the Stargate, giving Tau'ri a great advantage.
Sokar, who has amassed a wast army was about to challenge the System Lords, when he was killed in a mission by SG-1 and the Goa'uld resistance known as the Tok'ra (literally, against Ra). With Sokar Dead, Apophis, who was in his captivity, was able to usurp his forces and once again rise to power. He sought to make an alliance with Heru'ur, son of Ra, who inherited much of his father's power, but the treaty was sabotaged by the Tau'ri and the Tok'ra, leading to another dead Goa'uld. Pretty soon Kronus, a powerful Goa'uld was also killed by SG-1, who took over his mothership and used it to fight Apophis. Apophis was himself killed when he and SG-1 accidentally made contact with the foes of the Asgard, the Replicators. Eventually, SG-1 was able to lead the Replicators into destroying Apophis' ship, taking the Goa'uld with them.
With Apophis gone, the ancient Goa'uld Anubis returned. After he was exiled by the System Lords, Anubis became familiar with the concept of ascension, and found out that the Ancients now inhabited another plane of existence. He tricked one of the ancients, Oma Desala, into helping him ascended to their plane. The other Ancients, when they realized that Anubis was in fact evil, forced him into a lower plane of existence. But, in order to punish Oma Desala, did not cast him down completely, Anubis was still semi ascended, able to use any knowledge he gained through ascension, but none of the powers.
As a semi-ascended being, Anubis made his move on the galaxy. First he petitioned the system lords into allowing him to join their ranks by destroying the Tau'ri and the Tok'ra. He was able do give them massive defeats, but was eventually unable to destroy the Tok'ra (due to their guerilla ways) or the Tau'ri (because he still feared the Asgard to make a direct attack on Earth).
When this didn't work, Anubis turned on the System lords. He collected six ancient Goa'uld devices (which may have been Ancient in origin) and combined them into a weapon capable of destroying a galaxy. The system Lords, led by Lord Yu and Ba'al, attacked him, but, thanks to his Motherships enhanced with Ancient tech, and a new breed of warriors called the Kull warriors, which were immune to Goa'uld energy weapons, Anubis Dominated the System lords.
He soon took the fight to the Asgard, surprising Asgard supreme Commander Thor with his enhanced Motherships, but the Asgard were able to drive him off with their new ships.
Now sure of the Asgard impotence, Anubis prepared to make a direct attack on Earth. His Plan was thwarted, however, when Col. Jack O'Neill was able to use the repository of Ancient knowledge to divine the location of an Ancient outpost on Earth, and completely destroy his fleet with a barrage of Drone weapons.
Eventually, in 2005, the Rplicators invaded Milky Way, completely destroy the System Lord order. The Replicator invasion is marked as the end of Goa'uld empire in Milky Way.
However, Ba'al survived, and using clones of himself, fooled both the Tau'ri and the Tok'ra and evaded capture by both factions for some time.
When the final Ba'al clone was captured, Ba'al used a time travel device to alter history and make as though the Empire never fell, and crowned himself the Emperor of the Goa'uld. However, his plan was foiled when SG-1 who were not affected by the temporal displacement, managed to travel back in time themselves and kill Ba'al.
Finally, all the system lords of the Goa'uld were dead.
The Goa'uld are a parasitic race akin to small snakes (prompting Col O'Neill to coin the term snake-head, and other derogatory names). They have the ability to burrow into a host body (a Human or a Unas) at which point they would attach themselves to the brain stem, and completely take over the host.
A sure sign that someone is infested by a Goa'uld is that his would glow. Their eyes glow in times of stress, and apparently are used to reinforce or display dominance the parasite has over the host. It is unknown what biological mechanism they use to achieve that. The Goa'uld gives the host body peak fitness and a perfect immune system, as well as great healing ability.
A Goa'uld can enter a human either through the mouth or through the back of the neck. Entering through the mouth will leave no signs of infestation, and this is the method used when the Goa'uld wish to remain incognito.
Their ability to take hosts make them extremely effective spies and assassins, as they can leave a body behind and jump into a new one. This, in fact, is the method used by the Ash'rak, the assassins of the System lords.
Once infested, a Goa'uld cannot easily be removed. if in mortal danger, the parasite would release a deadly toxin into the host's bloodstream, killing them. The Tau'ri have no means of removing the Goa'uld without killing the host. The Tok'ra however, being Goa'uld themselves, have found way to remove the parasite without killing the host. The Asgard can easily just teleport a parasite out of the host.
Goa'uld reproduce through the queens. These Goa'uld produce young new larvae. These larvae are then implanted into a Jaffa, who carries it in his abdominal pouch. In turn, the young larva acts as an immune system to the Jaffa, giving him great healing ability. However, once a Goa'uld is mature, he will be taken out of the Jaffa and implanted into a human. The Jaffa, no longer having an immune system, will die shortly after.
An interesting feature of the Goa'uld is their genetic memory. They have all the memories of their ancestors, which makes even the young Goa'uld extremely dangerous. A queen can choose to withhold her genetic memory from her offspring, creating "blank slates".
Another curiosity is the presence of Naquadah, a super heavy metal in their system, which reacts to other naquadah around them, making them aware of other Goa'uld or naquadah technology, such as the Stargate, in the vicinity.
This became a part of their physiology when they started to use the Sarcophagus. The Sarcophagus is a healing chamber capable of sustaining a person in it for an indefinite amount of time. However, it also disrupts the chemical balance in the brain, causing the person to grow paranoid, egotistical, and a megalomaniac.
The over use of the sarcophagus, coupled with their genetic memory, gives an explanation for the behavior of the Goa'uld. They are literally "born evil".
The Naquadah in their blood also serves as an activator for their technology.
The Goa'uld empire was lead by the System Lords, a group of most powerful Goa'uld who held wast areas of the Milky Way. This was a relatively feudal order, as the system lords would regularly fight each other to increase their own power, but would come together under external threats, such as the Asgard.
The system lords commanded hosts of other, lesser Goa'ulds in their ranks, many of whom would attempt to assassinate them and gain their place. However, to become one of the System lords, you must have the acceptance of all other system lords, or else you would be considered a threat.
Each System lords commands wast armies of Jaffa, numerous Motherships, and many slaves seeded on numerous planets. since the rebellion on Earth, all writing has been banned by the system lords, making their slaves live in almost stone age conditions. The Goa'uld frequently use their technology as a sign of their divine power.
Goa'uld tech is far superior to that of present day Earth, but still inferior to that of other Ancient races, like the Ancients, Asgard, Furlings and the Nox (the races of the great alliance).
Their technology is based primarily on the element Naquadah, which the Goa'uld go to great lengths to get.
Goa'uld technology is highly stylized, usually being hidden from site, in order to create an illusion of "magic". This is in line with the Goa'uld claiming that they are gods and that they have divine powers.
Known Goa'uld technological pieces are:
A list of Goa'uld system lords:
The Tok'ra are a splinter group of the Goa'uld, formed by the Goa'uld queen Egeria. They abandon the ways of their brothers, instead seeking to coexist with the humans. Because of this they are the most wanted beings in the Goa'uld empire, and have developed wast stealth technologies to survive. Rather than forcing themselves onto a host, they only take hosts who are willing to be blended with a symbiote.