Buffy Summers is the vampire slayer one girl in all the world destined to fight the vampires.  Her watcher, Rupert Giles trains her to fight them.  Her two best friends, Willow Rosenburg and Alexander "xander" Harris help her kill them.

Cordelia Chace is the stuck up popular type who is dating Xander.  Oz is Willows boyfriend in the "phases" episode he was turned into a werewolf.

Last but not least we have the really cute Angel, formerly Angelus, who was Buffy's boyfriend but the complicated thing is he's a vampire (with a soul). They had sex and the gypsy curse that was put on him 80 years ago was taken from him because he felt one moment of true contentment.  Then evil Angel comes up with a plan to send the world to Hell seconds after he opens Willow completed the curse (he got his soul back). Buffy had to stab him to close the vortex, it closed but trapped him in Hell and the season closed off with her on a bus out of Sunnydale.