Once fallen soldiers and warriors cursed to un-life, Revenants are fast, strong, tough and generally not very happy, having been denied their final rest and everything. Revenants are generally either created by necromancers after death, or cursed to become revenants by witches while still alive. On rare occasion, a revenant will become as such because they possess an unholy item upon death, on even rarer occasion, a revenant will come back by hi own means, through sheer force of will, often to exact vengeance upon their murderer. Regardless of how a revenant comes into existence, however, there are a few fundamental things that stay the same. Firstly, the aforementioned boost in strength and speed, as is the case with many creatures of the night, Revenants simply outclass human in all aspects of physical performance (of course, their former training as warriors doesn't hurt).
Secondly, all revenants have free will due to the fact that they still possess a soul, this is what separates them from a necromancer's reanimated corpses or 'zombies'. They keep all of their mental faculties, and most of their memories from life.
Thirdly, revenants can only be 'killed' one of three ways, either destroy the brain, or rob them completely of the unholy energies that tie their souls them (Only someone with full mastery of death magic, like an elder litch could do this), or, conversely, expel the energies sustaining them from their body by subjecting the revenant to a large dosage of holy energy (weak healing spells won't have any effect on them). needless to say, they can be burned to death, but that's only if the fire manages to destroy their brains.
fourth, all revenants now possess the ability to learn magical skills, what skills they learn is completely up to them and the only school of magic they cannot touch is holy magic (obviously).
Lastly, and perhaps most importantly. all revenants are essentially the spiritual equivalent of a vampire. They must feed on souls regularly to maintain themselves. Not feeding won't 'kill' them, but the longer a revenant goes without feeding, the weaker they become. eventually they will be reduced to a mummified-like state where they are unable to move or speak but are still semi-conscious, plagued with an unending sense of famine until some unlucky soul wanders close enough for them to feed again. In this state, revenants are especially easy to destroy as they can not use any of their powers.
it should also be noted that most revenants learn to control their feeding so as to not kill when they do so and only partially suck out a victim's soul. Also, when they feed, any injury the revenant will have suffered will start to quickly heal, on top of their energy reserves replenishing. (Credit to Buio DiAngelo)