Not much is known about the enigmatic Engineers. All that is known is that the Engineers were incredibly advanced genetic engineers who made the Xenomorphs. It has also been shown that they played an important role in influencing the genetic development of mankind and that without them, the human race might never have become a race at all.
The first known contact between an Engineer and the human race happened in 2093 on LV-223. Four years prior in 2089, Archaeologists Elizabeth Shaw and Charlie Holloway discovered star maps in multiple locations that pointed towards LV-223 as a destination. The Weyland Corporation funded an expedition to LV-223. With the help of an android named David, Peter Weyland awakened the sole Engineer on the planet and attempted to communicate with him/her. The Engineer promptly responded by deciding to massacre the greeting party and attempted to fly one of the ships on the planets to Earth in a plan to either exterminate or mutate the human race. Fortunately and sadly, the crew of the ship Prometheus sacrificed their lives and their ship to disable and ground the alien ship. The Engineer then attempted to kill the sole survivor, Elizabeth Shaw, but fell victim to an abomination that Shaw gave birth via c-section. The abomination impregnated the Engineer who in turn gave birth to a Xenomorph-esque creature that burst out of its chest.
The second appearance of the Engineers in human history happened in 2122 when the crew of the Nostromus intercepted a distress signal that was emanating from LV-426. There, the crew found an Engineer ship. The Engineer piloting the ship was already deceased and cause of death seemed to point towards a grievous chest wound. The cargo of the ship was hundreds, if not thousands, of Xenomorph eggs.
Culture and Attitude of the Engineer Race
It is implied that Engineers are an incredibly proud race that tend to look down on other races to the point that they have no qualms about committing a planet-wide extermination as shown by the actions of the Engineer in the film Prometheus. The written language of the Engineers is noted as being extremely similar to the Mesopotamian cuneiform - so similar in fact that the android David that joined the expedition to LV-223 was able to interpret their language. This striking similarity between the two languages must mean that the Engineers made contact with humans long before 2089, explaining how our modern ancestors knew how to draw multiple star maps in caverns and suggesting that the Engineers may have not always been so aggressive towards the human race if they were willing to give humans the gift of written language. However, they may have also given humans the gift of written language simply as part of some unknown experiment.
Most Engineers seem to be 7+ feet tall and in tremendous physical condition. How much of this is natural or due to tampering with their own genetics is unknown. Underneath their helmets, they look incredibly human-like and have pale, slightly blue skin. They are able to breath oxygen just like humans, as shown by the supply of oxygen on their ships and the fact that they were able to visit Earth very early on without any negative repercussions or the need of any filtration equipment.
A majority of Engineer technology seems to be genetically based. Almost all of their technology seem to be bio-engineered as it all appears organic in nature, but it is unknown if it was truly bio-engineered or if it was merely an aesthetic choice by the Engineers.
Relationship with the Xenomorphs
The Engineers are presumably the creators of the Xenomorphs, and based on the almost fossilized appearance of the Engineer found on LV-426 in 2122, created this scourge a long time ago. Evidence seems to suggest that the Engineers created the Xenomorphs as the perfect bio-weapon. It appears that Engineer ship found on LV-426 by the crew of the Nostromus may have been some sort of "bio-bomber" that was on its way to disperse its lethal cargo against an unknown enemy before the Engineer aboard the ship ran into some unknown complications and the ship landed, or crashed, onto LV-426.