A vampire must feed upon the blood of the living in order to survive. Once ingested, a Kindred's body transmutes the blood into a more potent form, known as vitae. Vitae takes on a mystical quality, becoming much more than just normal blood. Vitae sustains a vampire's body, for with- out it, she would wither away.
Vitae is used to fuel multiple supernatural powers available to vampires. From Regeneration, to mystical Disciplines, the phrase "Blood is Life" holds especially true. Feeding upon the blood of the living or recently deceased is the most common means of replenishing your supply.
Review the help file for vampire for additional information.