Buffy the Vampire Slayer was (until the end of the 2003 season) a television show based on the movie of the same name. It starred Sarah Michelle Gellar as the title character and was created by Joss Whedon. The show follows Buffy and her friends, casually known as the Scooby Gang as they stop various small and Big Bads, usually in some sort of analogy for real life, especially high school.
Buffy Summers is an average girl, more or less. Or at least, that's what she wants to be (at first). But she's been called to be a Slayer, she who stands between the world and the forces of darkness, be they vampires or demons or even pesky mortals. Granted with super strength and agility, she spends her days at Sunnydale High School and her nights fighting evil.
She's allied with an ever-changing group, but the three that have always been with her are Willow Rosenberg (played by Alyson Hannigan), Xander Harris (played by Nicholas Brendan) and her Watcher, Rupert Giles (played by Anthony Stewart Head). There have been others who have helped her, including Angel, Cordelia, Oz, Tara, Anya, Riley Finn, Dawn, and Spike.