Cainites, as you might expect, descend from the Biblical Cain, the first murderer, cursed by God to walk forever in the land of night. Or such is the legend. There is no provable evidence of this fact that can be produced.
Perhaps more worrisome are the legends of the most ancient of the line, dating from before the Biblical flood: the Antediluvians are thousands of years old and may have been the first members of the modern clans. Many suspect that they have been in torpor since before Rome, and that their waking would be terrible for modern Kindred: some suggest that their hunger can only be sated by the blood of their offspring, others that they are completely inhuman and dangerous for both the Masquerade and the lives of all Kindred, and still others that even the most benign awakening would completely unsettle Camarilla society as they resumed control of their bloodlines.
A Cainite is created when a human is drained of blood unto death and then given vitae, the unearthly blood that flows through the veins of other Cainites. Some time later, the corpse arises as a Cainite; sometimes immediately, sometimes not for several nights.
All Cainites are corpses animated by the power of the blood, can expend it to achieve supernatural ends, and must replenish it by feeding on the living. Most Kindred fall into a death-like slumber during the day, at which time they are indistinguishable from a corpse save that they will be horribly burned by sunlight. An active Cainite still resembles a corpse to many forms of detection: she is room temperature, has no pulse, and takes no breath nor makes many of the obvious, unconscious signs of life that humans naturally produce. A Cainite can expend blood and learn theatrics to temporarily overcome these signs.
And overcome them she must, for the most important rule of the Kindred is the Masquerade: humanity must not be allowed to know that Kindred walk among them, for the forces of humanity united against parasites in their midst would inevitably destroy the undead.
From this follows several other Traditions, the second of which is Domain: the city is the rightful realm of the Prince, and other Kindred may claim territory within this domain as granted by the Prince. None may claim or contest lands within the city without the assent of its ruler.
The third Tradition is Progeny: only the Prince may allow another Kindred to embrace a childe, and childer created without this permission can be summarily executed (along with their sires, should the Prince desire). Only the Prince of the city has final say on the danger of bringing another Kindred into the world.
The fourth Tradition is Accounting: a Kindred given permission to embrace is responsible for the childe until she is released by the Prince to stand on her own recognizance. Until this time, mistakes made by the childe fall upon the sire.
The fifth Tradition is Hospitality: Kindred rarely travel from their home city, and when they do they are obligated to announce themselves to the local Prince and obtain permission to stay within the city. Kindred not offered hospitality, at best, may be treated as rogue supernaturals by other Kindred within the city and may even find themselves the subject of the next tradition…
The sixth Tradition is Destruction: Kindred may not kill other Kindred that have been accepted within the city unless given the permission of the Prince. Often, the Sheriff is the one Cainite in the city with the authority to hand out such justice when it is called for. Sometimes, if a Kindred has caused enough offense, she might be subjected to a Blood Hunt by the Prince, where she becomes fair game for all others within the city to catch and slay for the good of all.