In the Beginning, when God prepared the Earth for Man, he overthrew the minions of Satan. The movie has Merrick saying: "Let Satan tremble: the Slayer is born." Unless he was just using Satan as a metaphor, this implies that Vampires are Satan's spawn. We know Merrick is religious because he believes he will go to Heaven after all the Vampires are dead. Therefore, the Vampires and the Slayers must be part of the Eternal struggle between God and Satan.

So God banished the demons from Earth, but their legacy remained, since one Demon sucked the blood of a human before leaving, and the demon-soul spread from human to human through contaminated blood, as the infected one bit others. To counter this, God sent the Slayer. One girl in all the world with the strength and skill to destroy the Vampires, to stop the spread of their evil and the swell of their numbers. To each generation, only one is born, and she is given the power to defeat the forces of darkness, and the Watchers are given to guide her.

There are two forms of Watcher. One is reincarnated, with the knowledge of his purpose given from birth. The other is a normal human, whose duty is passed from father to son, from mother to daughter, and they study to learn the ways of the Evil Ones. Either way, the purpose of a Watcher is to train and advise Slayers: but never to interfere directly.

Now, before we go any further, a few things must be established:
1. Merrick was a Watcher. Now, he never referred to himself as such, but the opening voiceover in the movie says that the Slayer is "trained by the Watcher" and we see Merrick there and both he and Rupert Giles have much the same role, and they use the same spiel to Buffy about Slayers and their duties. So even though Giles is not reincarnated (as far as we know - its possible, although we know that he is not born with the knowledge of a Watcher like Merrick was) he and Merrick are both probably Watchers, but of two different types.
2. There is more than one Slayer. From the movie, we know that Buffy is reincarnated time after time. One girl is chosen by God to be inhabited with the soul of a Slayer, and to live her life fighting the Powers of Satan. We also know that Slayers have been around since the dawn of time (Giles says that "for as long as there have been Vampires, there has been the Slayer" and he also said that Demons left the Earth just as Man arrived) and we also have evidence that Buffy has only been around about a hundred times. The flashbacks in the movie tell us that Merrick has been Buffy's teacher in the past, at least four or five times (since he knows at least this many incarnations). It is likely that he and Buffy have always been paired together. Now, he also claims to have trained girls to be Slayers for a hundred lifetimes. If one Slayer is born every generation, Buffy can't be the only Slayer going around (take a generation to be thirty years - even though Slayers don't seem to last into their twenties - and multiply by a hundred. Do you get hundreds of thousands of years? I don't think so...) This is further supported by the fact that there are other Watchers. Giles received the title from his father, and his mother was one before him. Buffy wasn't alive then, so there must have been another Slayer.

Okay, so a Slayer was born to combat the evil. When she died, another was born. After several had tried, the first was reincarnated again. Most likely, to face the very Vampire that killed her. We know that Buffy has been fighting Lothos in many of her incarnations, so it seems likely that each Slayer is given a specific Master Vampire to combat, and is born in the country that he is in. When they defeat him, they may go to Heaven finally, or more likely, move on to face another Master Vampire, as there are plenty to go around. This may mean that Buffy is horning in on another Slayer's territory in Sunydale. The Master could have been for another girl, but Buffy is gunning for him anyway.

The Watcher has a complicated task. The kind who is associated with a specific Slayer (like Merrick) is not allowed to interfere physically, even if he feels that this time, the girl is truly exceptional. It is forbidden. Merrick's attempt to do so in this lifetime got him killed, and endangered Buffy. We can only speculate what reprimands the One Above gave him. That might be interesting to find out, actually...

So the Watcher must seek out the girl (he is probably born near her, although a few decades before she is), recognising her by the Mark of the Coven, and to teach her the ways of the Chosen One, and to prepare her to face evil - and in particular, her personal enemy.

Giles' type of Watcher is, oddly, more active and more passive. He is free, it seems, to physically intervene (although he hasn't the skill that a Slayer has) but he is not given a specific Slayer to train. I can only assume that God placed the Watcher order on Earth as a recorder of information, to inform Merrick's kind of Watcher, and in case of an emergency, as happened in Buffy's case. If need be, a member of the Order of Watchers (my name for the normal human kind - like Giles) may step in and act in the stead of High Watcher (my term for Merrick's type) in training a Slayer personally. Normally, a Watcher of the Order will probably live out his entire life without actually meeting a Slayer, although I assume he studies them and records information about them. Probably, a Watcher will try and learn more about Vampires to help future Slayers and Watchers, and record his findings in a book to be passed down through his line, in the hopes that it be used successfully one day (in fact, we know Watchers read the diaries of previous Watchers). This life is probably harder to stick to, as the lack of direct influence on Slayers would lead you to believe you could ignore your duty most of the time, but this is far from the truth. As we have seen, ancient texts and the writings of people long since dead have proven very useful indeed to Buffy in Sunnydale.

So, as the Vampires attempt to populate the Earth anew, preparing the way for their progenitors to take the Earth again, the forces of God move silently through the years hacking away at them, piece by piece, doing their best to stop the Evil. It only remains to see who will win in the end.

It has been speculated that the Watcher Order may have another role. While the High Watchers train the Chosen Ones, it may be that Order Watchers select promising ordinary human girls (or boys) to be Slayers. This would indeed make sense. When the Slayer dies, there are several years until the next girl is old enough to perform her duties, and the Vampires will have free reign for a while. In order to counter this imbalance, the Watchers may have several Pseudo-Slayers standing by to fill in until the real Slayer is born. It may even be, in fact, that these Pseudo-Slayers become the mothers of future Watchers (or Slayers). At the moment, this is purely speculatory, but quite possible (indeed, probable). Further research must be done to discover more.

One interesting fact about Vampires that is often ignored is the fact that Vampires are taught while they lie dying. The demon soul infecting their "dead" body informs them of certain things. This is because the vessel of the demon soul is no longer human. It may have the likeness, the memories, even the personality of the human it takes over, but a Vampire is a demon at the core. This demon soul is somehow aware of things the Vampire who donated the blood knew.

The false Annointed, whom Buffy slew on her date with Owen, claimed that "they told me about you while I slept", referring to Buffy. This is evidence of the demon soul transferring knowledge. This is the counter to the fact that Slayers often retain some knowledge of their past lives. To balance this, the Vampires have their own way of transferring knowledge with procreation. The limitations of this knowledge transferral is unknown at this time.

Throughout time, there have been certain Vampires of note - ones who deserve special attention in a historical volume such as this.

Lothos was one such Vampire. He lived for centuries, and was one of the most powerful ever to have lived. The only thing ever to cause him despair, was one particular Slayer. She whom we now know as Buffy. And even she didn't bother him too much.
Interestingly, Lothos was able to maintain facial hair, something not many are able to do. Of course, he was probably not able to grow it, but it was still quite a feat. In the Dark Ages, he wore a thick, goatee beard. By modern times, he had shaved this down to a moderate moustache.
At some point, Lothos decided to sleep for a time, and his slumber lasted decades (perhaps centuries). His right-hand man (no pun intended) Amilyn was to arrange the great bloodbath which would feed him on his awakening. This was most likely some kind of ritual. The slumber perhaps lengthened his life, or increased his power, while the mass murder which followed replaced any energy he may have lost during his sleep. It is likely he had performed this any number of times, since he was very old and strong.
Lothos met his long-overdue death at the hands of Buffy. In the end, it was his overconfidence that betrayed him.
Also refer to: Lothos, a character study.

Another important immortal, whom we know very little about, was Aurelius. This Vampire was bestowed with the rather unique gift of prophecy. In the 12th Century, he began to record the visions he saw in a great volume. When others saw this, they were intrigued, and many of the undead became his followers, the so-called Order of Aurelius. The brotherhood followed his teachings, and took as their symbol the sun, surrounded by three stars. By this, any Brother would know another.
It is unclear whether Aurelius is still alive or not, but the Brotherhood is still very much so. They recently have been involved in the fulfilment of one of the prophecies of their founder.
The Annointed was raised from the ashes of five. He was prophesied by Aurelius to come, and come he did. In the form of a little boy. As more on this unfolds, it shall be recorded herein.

When brute strength is needed, Vampires can always call on the Three. These are three extraordinarily strong Vampires, who act as mercenaries, doing the dirty work of other Vampires, for who knows what reward. They no longer exist, however, since they failed to complete a mission, and offered their lives up as atonement. The offer was accepted.

There are many more noteworthy Vampires, and I shall continue to research them.

The information contained herein was gathered from the movie and the series and includes my own speculation. It will continue to be updated as the series reveals more about Vampiric history and the role of the Slayer. Information from episodes unaired in the UK obviously has not entered into my discussion, so it may be out of date in America. I apologise for this. It will change regularly.

Remember to read books on this subject. Do not read merely from the screen in front of you. Computers do not smell. Smell is the most powerful of all the senses, it can bring back a memory with perfect clarity. Books smell musty and rich. The knowledge gained from a computer has no texture or context. It is there and then gone. If it is to last, the getting of knowledge should be tangible. It should be... smelly.
Thanks to Watcher Rupert Giles for the above wisdom (I paraphrased a little). His work is some of the foremost on Vampires and their history. Definitely suggested reading.

Another note of thanks to the "Guestbook Spoiler", Mr Rick Yunker, for his research and speculation on Pseudo-Slayers.