The Story
"Into each generation, a Slayer is born. One girl in all the world, a chosen one. One born with the strength and skill to kill the vampires, to stop the spread of their evil and the swell of their numbers."
It is not only vampires, however, that these warriors have been chosen to annihilate. They also must destroy countless other types of demons and unholy creatures. With the aid of a Watcher who trains and looks out for the Slayer endowed with superhuman strength who have been known mostly to lurk in the shadows alone, following their prey - the bloodsucking undead. As you can imagine, these heros have an unhealthy expiration date... most don't live to see past 20.
Welcome to Sunnydale...
Essentially ground zero for all things unholy, Sunnydale, California, was build directly on top of a Hellmouth: a portal to a rotted dimension that just about every slag wants to unlock and unleash hell on Earth.
Now with the aid of her Watcher Giles and her loyal friends (or "slayerettes" or "scoobies" as they have named themselves) Willow, Xander, Anya, Tara, Spike and her sister Dawn, she has battled and defeated many foes including The Master, The Anointed One, Adam and Glory.
The characters of Anya and Dawn didn't come around until the last half of the series through not so normal means. Anya is over 1100 years old. Although she was born human, she was made a vengance demon by the powerful demon D'Hoffryn. After losing her powers as punishment for a mistake made, she became a human again and joined the scoobies and fell in love with Xander.
Dawn, although she is 16 years old, was actually created only about two years ago. She was created as a vessel to hold a source of great energy called "The Key" that would open all the doors between dimensions and bring down the walls inbetween them creating unreversable choas. She is fully human, however. She was created my monks who were protecting The Key from Glory who wanted more than anything to bring choas. They placed the energy in the form of a human sister of Buffy because they knew the Slayer would protect Dawn, The Key, with her life.
One's strength can only truly be judged by their conviction and dedication to their calling, their prophecy, their fate and their gift. Proving that she was the truest and strongest of all slayers, Buffy gave her life to obstruct Glory for the Dawn of our world's salvation. Although the slayer lore warned that most slayers don't live past 20, Buffy was an exception... but not by much.
It's a grave new world. Prepare yourself for ressurection and retribution.
For more of the Story, check out the Seaons.