blood

Blood is the sole dietary need of a vampire—whether the vampire drinks animal or human blood. It is important for the vampire to consume it regularly, as the blood strengthens them both physically and mentally; it makes them more capable of rational thought and action.  
The blood of both animals and humans varies largely in flavour. Much in the same way that humans have preferences for food, vampires may have preferences in the blood they drink. Their preferences may be indicative of their hunting styles and personalities. Usually, the larger the animal, the better tasting its blood will be—predators are the best. We are told that Emmett has a preference for grizzly bears, and that Edward prefers to drink the blood of mountain lions. Of course, though, the blood of animals doesn’t really appeal to vampires.        Human blood is what vampires truly crave, but some “vegetarian” vampires choose to drink only the blood of animals. Although it doesn’t completely satiate the thirst, it keeps them strong enough to resist drinking the blood of humans. 
There is a vast range of types of human blood available to vampires, and some are more potent to different vampires than others. Bella is considered Edward’s “singer,” because her blood has a scent that is “appallingly luscious” to him, more than any other vampire. Her blood “sings” to him. The temptation is nearly impossible to resist, so it is very easy to succumb to the temptation, especially if a vampire is not vegetarian, or has not been a vampire for very long. This kind of scenario, where one human’s blood smells more potent than another’s, seems to be rare—Emmet has come across this only twice. For Edward, Bella is the first human to whose blood he reacts so strongly to. Other vampires, like Jasper, seem to be oblivious to the variations in smell or flavour of human blood.  
Some vampires, like Carlisle, may be capable of dealing with human blood without seeming to experience the natural desires of a vampire. Carlisle has trained himself over a long period of time to resist the urge to drink human blood. His methodical practices have served him well over the years, and he is now able to complete his focused work as a surgeon and a doctor with ease, and without danger of drinking his patients’ blood. Carlisle has never tasted human blood.
When a vampire draws blood from a human, it usually results in a draining of blood from the body. Once a vampire begins to drink the blood of a human, it is almost impossible to stop. This explains why so few vampires are successfully created. Once a vampire tastes, or even smells blood, it becomes very hard to keep from feeding—sometimes impossible.