Much of this was already posted in my personal journal, so please bear with me.
What is a Vampire?
A vampire is not a walking corpse, a descendant of some godly being, or an evil being created by Satan. (Though note that some vampires believe the story about Lilith, just as some humans believe the story of Adam and Eve.) Vampires are just a race of creatures - that is, a species which is at its basist a mutation on the human gene. As such, vampires are animals like any other - subject to certain rules and limitations, but living creatures. They breathe, they interact with their environments, they hunt. They can be killed.
What's up with the bloodlust thing?
Vampires need blood. They can eat and drink normal things (and it's recommended to keep you fit and strong), but blood is what they really need. Without blood, their "supernatural abilities" start declining, they weaken, and eventually, they go mad. Vampires can not go nap in a coffin for a couple centuries with no blood. (Take that, Anne Rice.) They don't need to feed terribly often - one completely drained victim a week would be enough. Since most vampires prefer not to kill their victims (if only because that makes it difficult to live comfortably in one place), it's advisable to break this into two-four victims every 8-10 days or so. Really, the amount you feed varies greatly from vampire to vampire, and it's up to your discretion. A minimum amount would be perhaps a glass of blood every week or two. Without even that much, you would start to deteriorate. The type of blood does not matter much, but it does have some effect. You would not get the same strength from the blood of, say, a parakeet (MIR) as you would a human. This varies depending on the animal's size and whatnot. The closest you could get to human in terms of goodness for vampires would be a wolf's or big cat's. Another large predator. The blood of farm animals - cows, pigs, what have you - is of medium value, but extremely useful in a pinch.
It is worth noting that more important than the physical need for blood is the desire for blood. A vampire that refuses to drink has a great deal of self restraint, because the longing for blood is all-consuming and incredibly powerful. That's why vampires who've been "blood-starved" go mad.
What else? Abilities. VAMPIRES CAN NOT FLY. There will be no Crouching Vampire, Hidden Werewolf mumbo jumbo. They can, however, do some other neat stuff. Unfortunately, though, they are also bound by some pretty annoying rules.
What supernatural abilities do vampires have?
Most vampire share some common abilities, like increased stealth and an ability to move quickly and silently. (Note that some vampires may not share in these "common" abilities.) There are also certain abilities which are uncommon, and individual vampires may have certain powers unique to themselves.
Common powers are fairly straightforward. All vampires are, generally speaking, immortal. They can be killed only by "total destruction" (such as beheading, being totally incinerated or crushed, or having something significant shoved through the heart, bursting it) or by breaking one of the rules. They have incredible healing powers, as most wounds will heal within a matter of seconds. They can NOT regenerate body parts; however, body parts that have been removed will stay "fresh" and not decay (if they are left untouched) and there have been instances where a severed finger or the like has been successfully found and reattached several centuries after it was lost. Additionally, almost all vampires have heightened senses and greater-than-human strength. They can't necessarily pick up cars, but they can run faster, jump higher, hold their breaths longer, punch harder, whatever. Note that generally a vampire's abilities will grow stronger the longer he's around, so older vampires tend to be much scarier than newer ones.
Individual abilities are more nebulous. There are reports of vampires that can control or turn into mist; vampires who can go invisible; vampires who can effect the weather; vampires who can perform limited telepathy or telekinesis; vampires that can control animals; even vampires that can turn into certain animals.
What're the rules on daylight?
Vampires are nocturnal. Going out into direct sunlight will not BURN THEM INTO A FLAMEY CRISP (OMG DRAMA), but it will be HIGHLY UNCOMFORTABLE (even fully covered) and direct exposure can badly burn new vampires. Their skin is extremely sensitive to sunlight, obviously. Very old vampires may have gained the ability to walk in the light, but they lose their supernatural abilities, including speedy healing, so they generally avoid it (or are extremely discreet about it).
So what constraints are they under?
You can't move about during the day, obviously. Although vampires don't HAVE to sleep all through daylight hours, they have to sleep as much as a human would (the number of hours depends on the individual vampire). Whether or not your vampire goes into a trance or deep sleep and whether or not he dreams is ENTIRELY up to you.
Vampires don't drink "dead" blood, but only in the sense of blood that's gone bad. Drinking blood from a blood bank is fine, as would be drinking FRESH blood from the body of a FRESH corpse. However, blood sours very quickly, and drinking "bad" blood would have the same effect as drinking bad milk.. It would make you very, very sick. As such, most vampires are careful to avoid dead things, though some like to push their luck.
Vampires can NOT enter a *home* unless invited first. Businesses and the like are another matter, and they may enter someone's *property* (though it's typically considered polite to do so only when going to the door to request entry). "Home" is loosely defined here, and there's no real enforcement. They just can't. If a vampire found a way to and his "council" found out, he would be harshly punished. It's just considered proper etiquette. (Note that that invitation, once offered, can NEVER be withdrawn. So be careful.)
Some - though not all - vampires are effected by other folklore. Some may feel compelled to count things they find in piles (grains of rice or barley, etc), and some may not cross lines of salt. Some may avoid crossing running water, and some may be offended by religious artifacts. (However, note crosses and holy water have no effect, really.) Please feel free to include your own limitations.
How do you become a vampire?
You can not be born a vampire. Vampires do not grow, and a female vampire can not become pregnant. A male vampire can impregnate a human, which is a situation we'll deal with later. First, I want to go over your everyday, full-blooded vampire.
You become a vampire by means of ingesting a significant amount of vampire blood. The simplest way to go about this is to drink a vampire's blood, though a transfusion would also work. If you get a small amount of vampire blood in your system, through a means such as a quick bite, infected blood on an open wound, sharing needles, etc, it will not make you a vampire. A very small amount would merely have some strange (and typically unique to the individual) aftereffects: eyes may glow silver at night, the person's immune system may strengthen (or weaken) slightly, etc. You don't actually get any vampire powers, but full vampires can smell that you've been "touched." If you get somewhat more blood in your system, but not quite enough to turn.. well, that's not good. Anything more than a splash and vampire blood works as a poison, and it will slowly cripple and kill you. (This could take as little as a few weeks or as much as a few years, depending on the amount.) For this reason, some individuals who have been "infected" may actively seek out vampires and attempt to get them to "turn" them, as an alternative for dying. (That's not the only reaction these "sick ones" have had, however.)
On the poison note: A vampire's blood really is quite toxic. It's typically enough to wilt flowers, though you won't see it eating through any tables. It is almost certainly fatal to any animal unlucky enough to ingest it, with a few exceptions (those creatures "kin" to the vampire, such as wolves, vampire bats, etc). It has a distinct (though not unpleasant) smell to those who can detect it, which is why one vampire can never hide from another. (Or from a werewolf, for that matter.) It's hardly toxic to other vampires, however - on the countrary, it can be quite.. intoxicating. Yes, vampires can literally get drunk off one another's blood. Though it has none of the same effects as drinking human blood (ie, it won't sustain you by any means), it can be a pleasurable experience and can distract you from the fact that you haven't had a bite in awhile (if, say, two vampires were stuck together somewhere). The effect for the bitten vampire is much the same as what a human feels, and it is somewhat draining - it is possible for one vampire to kill another in this method, and being drunk from does weaken you. For that reason, it's typically looked down upon somewhat (it's sort of seen as a practice of n00bs and drunkards) and is not encouraged. If you're curious as to wht your vampire would taste like, there's an adage comparing vampire blood to good wine - supposedly the longer you've been turned, the better you taste. (No you can not gain super l33t tingly powers by drinking an old vampire's blood, nor open up a psychic connection or any of that.)
How can you tell a vampire's a vampire?
Many animals - especially horses - are very uncomfortable and skittish around vampires. Most (though not all) animals can "sense" them. Most vampires can also "sense" others, though this ability varies by individual and by age/practice. Creatures like werewolves and shifters can generally detect a vampire by scent, because they don't smell human. (Though if one such creature had never encountered a vampire before, he wouldn't know what to make of the smell.) Humans can detect vampires through a number of ways, but it's generally rather difficult. A bloodtest will show vast abnormalities and imply the DNA is vastly mutated; however, since most people can not just do random bloodtests on people they meet, other methods have been developed by vampire hunters and those with an interest in the occult. A vampire can't enter a home without being invited, as mentioned earlier, and vampires do not cast shadows in natural light. (This means a streetlamp will cast a shadow, but the sun will not. Obviously, this rarely comes into play, as few vampires are out during the day.) Physical signs include things which are generally difficult to pin to a vampire specifically, like generally pale skin (a vampire that has not fed in awhile will become very easy to spot, as its skin begins to lose all pigment after a stretch and may even begin to look bluish or dead) or an unnaturally cold gaze. Vampires' teeth also become distended during and immediately after feeding (and in some cases, just before), so looking for big canines will help some of the time but not all.
Ways that will not help you to recognise a vampire include carrying around a mirror or digital camera (sorry, they have reflections and do show up on film), throwing holy water on people, carrying lots of garlic, and studying nails and teeth.
This FAQ is still a work in progress. If you have any questions, please ask!