A familiar is an animal or other small creature magically bound to a wizard. There is a telepathic link, but it also gifts animals with greater intelligence. The wizard in turn gains some abilities from the familiar.
Blackwing is V's raven familiar, which means that he can communicate telepathically with him over short distances, and gives him Alertness (apparently) when in close proximity.
Perhaps most importantly, the bond between them means that one's death or injury is detrimental to the other. If blackwing dies V loses all the benefits he brought plus some. If V dies, I believe blackwing is killed by the shock.