The Klyntar are a peaceful alien collective who seek to join with other races to become the perfect warrior of nobility. Only the perfect blend of moral and physical ideal can create such a warrior and anything less results in a bad pairing. A bad pairing will disconnect the symbiote from the collective and cause damage and horror.
The Symbiotes were scattered across the galaxy after their home planet was devoured by the world-eater Galactus. For some reason, Galactus could not siphon the life-force from the symbiotes themselves. This event left some symbiotes psychologically traumatized and they became renegades and disconnected from the group. Some of the race is now nomadic and live on various worlds throughout the cosmos.
The remaining collective formed a world in an uncharted area of space and continued it's mission. It would also continue to bring back those who had left the collective to repair them and remind them of their true purpose.
Symbiotes require an intelligent organism for a host. Once bonded, a symbiote can remove all poisons and cancers from it's host body. Without the perfect host to become a noble warrior, the symbiote disconnects from the collective and bad things follow.
Symbiotes in these situations have a give and take relationship with their hosts, but usually take more than they give. When Spider-Man was first bonded to the Venom symbiote during the Secret Wars, it seemed too good to be true: he no longer needed web shooters or web-fluid, he was stronger, and he could morph the symbiote at will between street clothes and his costume. The negatives quickly became apparent, however, as Spider-Man became more violent and was constantly exhausted. He discovered that this was because the alien was taking his body out on patrol in his sleep. Spider-Man rejected the alien and it found its way to Eddie Brock. Since then there have been numerous other symbiotes; most are relatives of the Venom symbiote who were born on Earth.
Symbiote biology is a bit conflicted between generations, but the seemingly undisputed facts (except in the case of Anti-Venom) is that they amplify the physical strength, agility, and healing of their hosts, are vulnerable to sonics and fire, reproduce asexually, and seemingly grow stronger with each generation. Symbiotes also have an amazing camouflage ability, and it seems they can absorb the abilities of previous hosts (having bonded with Spider-Man, it gave later hosts wall-climbing abilities).
They are empathic, usually drawn towards negative emotions, and feed heavily on the brain-produced chemical phenethylamine. The loss of phenethylamine is the very thing that leads the hosts of the symbiotes to become violent and mentally unstable, and is the largest drawback to being the host of one.