From the Pad of Definitions
From 1.01 Pilot (episode)
A representation of the cross on which Jesus was crucified, representing the goodness of Christ and God and the power of good over evil. Crosses are used to ward off vampires and demons that are associated with Satan. Much vampire lore insists that the one holding the cross must have faith if it is to work properly as a deterrent.
A cross isn't usually powerful enough to kill a vampire or demon on its own, but it can drain them of strength, burn their flesh, and keep them at a safe distance.