Buffy Summers was “activated” as the Slayer when she was 15. After burning down her school gym to defeat some vampires, she moved to Sunnydale, the location of a Hellmouth. She repeatedly broke with tradition, cutting ties with the Watchers Council, falling in love with vampires and, finally, making sure she was the last “chosen one.”
After Buffy temporarily died when she was drowned by the Master, a new Slayer was activated – a Jamaican woman named Kendra. She was far more studious than Buffy, though perhaps not as adept in the field. She was killed only a year or so after her activation by Drusilla.
With the Slayer line now passing through Kendra, her death activated Faith, a wild child from Boston. After the vampire lord Kakistos killed her watcher, Faith fled to Sunnydale where she was eventually corrupted by her Slayer powers and joined forces with the Mayor. She redeemed herself later by fighting alongside Buffy in the Battle of the Hellmouth.