Buffy grew up in Los Angeles, a happy, popular (and impressively shallow) girl. All that changed, though, when she was fifteen years old, and a man named Merrick informed her that she was The Slayer. The one girl in all the world with the strength and skill to hunt the yadda yadda... she hates that speech. Small wonder, because it turned her carefree life completely upside-down. In addition to seeing her parents' marriage crumble while she fought for her life on a nightly basis, her grades plummeted, her friends shunned her, her Watcher (Merrick) was killed, and she was expelled from school for burning down the gym. Which was a valid choice, what with it being full of vampires at the time, but she couldn't exactly share that bit of info with the school board.
She arrived in Sunnydale with Joyce, who was determined to make a clean start of it, but that stupid Destiny thing wouldn't leave Buffy alone. When she walked into the Sunnydale High School library in search of some textbooks, the librarian (one Rupert Giles - you may have heard of him) confronted her with an ancient tome inscribed 'Vampyr'. So not the book she was looking for. She insisted that she was retired, but with The Harvest looming, she reluctantly accepted her duty. Fortunately for Buffy, she found a group of friends to share her burden. Xander and Willow dubbed themselves her Slayerettes, and along with Giles (and a certain handsome vampire named Angel), they made sure that Buffy wouldn't have to face that duty alone this time around.
Buffy tried to make the best of things while juggling slayage, school, and a protective mother, but she was bitterly disappointed to have to break things off with an almost-boyfriend named Owen when it became clear that dating a nice, normal guy while being the Slayer simply wasn't in the cards. Xander would have been more than happy to step up to the plate, but Buffy only had eyes for Angel. Except for her brief death at The Master's hands (and her subsequent lack of healthy coping), Buffy seemed about as happy as one could expect. Then her birthday came, she and Angel made love, and all was hell and badness.
After she had to kill Angel to save the world, Buffy packed a bag and ran away. She lived on her own in Los Angeles, trying to put her former life behind her, but when it became clear that she couldn't run away from who and what she was, Buffy came home. Angel returned, only to break her heart again, and the new Slayer she thought she could share a common bond with turned out to be a bitter enemy. While Angel lay dying after an attack by Faith, Buffy turned to the Council for help, but when they refused, she turned her back on them. And then she blew up the school.
Graduated from both SHS and the Council, Buffy started college at UC Sunnydale. Between her new-found independence and her new boyfriend, Buffy started to drift away from the Scoobies (who were all doing a fair amount of drifting themselves). She allied herself with The Initiative, enjoying the benefits of having a well armed tactical team at her back, but when their pet project went amok, it was the original Scoobies who came together to prevent Adam from raising an army of demon/human hybrids. Re-connected through the Enjoining Spell, they resolved to stay closer in the future, but once again, events conspired to throw Buffy's world into a tailspin.
Riley's strength failed him, Spike started stalking Buffy, and she had a fancy new Key of a sister to protect from an insane hellgod who wanted to rip open the very fabric of reality. Any of which would have been ordeal enough, but on top of it all, Joyce suffered a lingering illness and died, leaving Buffy solely responsible for Dawn. Buffy dropped out of school and tried her best to raise her sister, but when the only way to save the world was to close a portal with Dawn's blood, Buffy decided to give it her own. She jumped into the rift and was killed, dead before she reached the ground.
The inscription on Buffy's grave read "She saved the world. A lot." And you would think that was a fitting end to a Slayer's life, except that Buffy's friends weren't ready to let her go. Willow and the others performed a dangerous spell to bring Buffy back. Unfortunately, Buffy wasn't happy to be alive again because the spell had ripped her out of heaven. Depressed and angry, Buffy began a mutually abusive relationship with Spike. She kept it a secret from the others, but the violence and rough sex were the only things that made her feel alive. Regardless of Spike's original intent, the relationship culminated in attempted rape. After that, and after seeing Willow's own despair nearly destroy the world, Buffy resolved to put the darkness of the past year behind her and live again.
With a new job as a counselor at the rebuilt Sunnydale High, Buffy and her sister grew closer. But (and isn't there always a 'but...'?) things were doomed to fall apart yet again. Spike returned from a quest with his soul restored (and his head full of crazies), and The First Evil decided it was time to take over. Once again, the weight of the world sat heavily on Buffy's shoulders. Hardened and in over her head against The First, Buffy made some impulsive and rash decisions which distanced her from the rest of the group, but she proved herself in the end, saving the world with a bold plan that resulted in the Calling of all the world's Potential Slayers.
The Sunnydale Hellmouth sealed, the series ended with Buffy no longer the Chosen One. Instead, she's one of the Many now, and for the first time Buffy finally has what she's always wanted: the freedom to choose her own path.