Vampires feel an overwhelming need to drink human blood. Their thirst is only satisfied by indulging in it; generally vampires are less bloodthirsty after drinking; it is however more difficult to abstain from it in the future. This is the case with Jasper Hale.
On the moral grounds that all human life is precious, a very small proportion of vampires in the Twilight universe, the Cullen family and the Denali Coven, get their sustenance from animal blood, jokingly call themselves vegetarians.
Vampires usually move around alone or with a mate. It is said in New Moon that James' coven, which included a total of three vampires was considered large. The only exceptions are, in order of decreasing numbers, the Volturi, the Cullens and the Denalis. The vampires diet is reflected in the color of their eye: red for those who feast on human blood, topaz for those who drink animal blood, burgundy for those living on a mixed diet. Black eyes denote a hungry vampire, independently from their diet. According to Eleazer, abstinence from human blood helps vampires relate to other vampires and form bonds with them because it makes them less animalistic.
A vampire’s strength is also affected by the amount of human blood in their system. If a vegetarian were to drink from a single human, their strength would only increase fractionally. However, newborns, still retaining the human blood they had when they were alive, are much stronger than older ones; it takes about a year before both blood and strength decrease to their definitive levels.
The blood of each human has an individual taste and smell; sometimes a vampire finds a human whose blood he or she finds absolutely irresistible. The Volturi call these humans "la tua cantante" or singers, as their blood "sings" to that particular vampire.
Vampires can smell blood from far away and follow its scent, particularly if it has been spilled. Most vampires find it impossible to resist it when a human begins to bleed, and as a result it is very difficult for a vampire to stop drinking once started. It is somewhat easier when the vampire is a vegetarian, particularly if he or she has been for a long time; Carlisle Cullen is the epitome of this characteristic: he is shown to be completely capable of suppressing this urge.