Vampires in the Felneyverse can be classified into four types. All of them share the following common traits:

Immortality - Vampires can "live forever", or, at least, have a lifespan vastly extended beyond any normal human, no matter how healthy.

Resistance to injury - Vampires can either resist most common weapons, or at injured by them, but heal rapidly and "perfectly", experiencing no scars or lasting disability.

Blood lust - All three kinds are dangerous, and kill and eat victims, but for different reasons.

Type 1 - Viral

These vampires are infected with a virus which alters their bone marrow, and other morphology. The bone marrow is changed into a tissue which oozes out the immortality serum. This serum causes a human to undergo a slow, agonising change into an immortal. The virus re-writes other parts of the vamire's biology, too. As the bone marrow no longer produces fresh blood, the vampire is required to replace it themselves for energy. This is acheived by fangs, which they grow, and which have a connection to the bone marrow in their skull. The blood they take from victims essentially just flows through this, and joins their veins in the normal way. Any blood will do, as they no longer have any immune system. They no longer need an immune system, however, as their immortality replaces any cells destroyed by disease or poisons.

Viral vampires can consciously choose to use their fangs to inject immortality serum, and the vampire virus, into a victim, and convert them. It is possible, but unlikely, to save a victim if an antidote to the virus can be quickly administered. The "conversion bite" doesn't administer enough immortality serum to render a victim immortal, though it may cause some other effects, such as reversing their ageing somewhat, curing a disease or causing some scar tissue, or even a small amputation, to be replaced.

Viral vampires, though immortal, are still just humans underneath. If a major organ is destroyed, or they lose too much blood, they will die like anybody else.

Type 2 - Bacterial

Bacterial vampires are created by the same bacteria that creates zombies, but only by a very exacting dose. Too little, and it doesn't infect them, too much, and they just become a normal zombie. These vampires are essentially just zombies that retain (or, rather, can use) their intelligence. Or the intelligence of the living person they once were, depending on how you look at it.

Being zombies, these vampires have an uncontrollable drive to both eat living flesh, and to propagate their own kind. Anybody bitten by them will almost always just become a normal zombie. They have no way of "controlling" zombies, or anything like that, but they are also not attacked by them.

These vampires have the same "immortality" of zombies, but enhanced. Their biological systems continue to do an impression of life, pumping blood around and keeping the tissue "fresh". Otherwise the biological processes have stopped, and the body is completely infested with the zombie-creating bacteria. Like zombies, the only way to kill them is by destroying the brain. They can be immobilised by severe damage to elsewhere on the body too, though.

Type 3 - Magical

These vampires are most in-line with central European folklore. They are magically immortal, are revived by moonlight, sustain themselves with blood etc. They are also capable of shape-shifting to some degree, becoming bats or other creatures. They are hurt or killed by sunlight, as it is the holy light God gave to the world. They also can't enter churches, and are scared of crosses. They can be bound or killed by destruction of the heart, often with ritualisic and magical aspects added to the act. These tend to exist in the Delta Universe.

Type 4 - Magitek / Jenova

Only in the Theta Universe, Vincent Valentine is one of these. He was injected by Jenova cells, derived from genetic material from preserved "Ancients" (Cetra). The degraded, but still magical, nature of the cells twisted his body into a new form. He looks normal, but can shape-shift into a number of bizarre creatures with different magical powers. He can also partially shape-shift, growing only wings or fangs, for instance. He is able to use the life-force contained within people's blood to heal even the severest wounds.