Willow Rosenburg is Xander Harris's best friend. When Buffy moved to Sunnydale she became best friends with her as well.
Willow had a crush on Xander ever since Buffy moved to Sunnydale. Sadly Xander never noticed her and she gave up on him. In season 1 Willow had a small love intrest in the form of a demon over the internet which she had unleashed using a scanner. Giles and Jenny contained the demon.
In season 2 Willow met Oz, they were the only two pupils in the school to be chosen for a computer company in the career fair. Oz first saw Willow when she dressed up as a rocker babe ghost for Halloween.
Oz and Willow started dating, and even when Oz found out he was a werewolf Willow still liked him.
Willow started studying witchcraft, with the help of Giles and was able to to the spell to curse Angel with a soul again. She did the spell too late however, and Buffy had to kill Angel.
When Angel came back from hell and Buffy hid that piece of news from her friends, Willow wasn't angry with her like the rest of the scoobies, she understood secrets because Willow herself had one-- Xander and her had kissed!
Xander had finally noticed her- too late it seemed, because both Willow and Xander were taken.
When Willow and Xander were captured by Spike, they kissed in the heat of the moment- and Oz and Corelia were the ones to rescue them. Oz forgave Willow, but Xander was not so lucky.
In season 4 Willow starts at college and everything is normal until Oz starts noticing another girl. Veruca is her name and she is a werewolf too. Unlike Oz, she embraces the wolf and runs free. Oz, not wanting to let her harm anyone, keeps her in his caze with him at night. When Willow comes to bring Oz food, she finds the werewolfs have mated- twice. In his wolf state Oz kills Veruca but then decides he should leave and explore his wolf nature. Willow is heartbroken.
She cries and drinks over Oz, eventually casting a spell to make her will be done. It goes wrong, and she is asked to be a vengance demon by d'hofryyn, the demon who created Anyaka all those years ago. She declines, and the spell ends.
At wicca class Willow meets Tara, a powerful witch, and they fall in love. The hardest thing is when Oz returns, wanting Willow back. Willow's pain is huge, she now loves Tara. Oz leaves again- maybe forever.
In season 5 Tara's parents come to take her home- they think all the women have a sort of demon inside them. Spike proves this wrong, by hitting Tara- (it hurts him with the chip so she must be human) and Tara stays and celebrates her birthday at the bronze with the gang-she's now one of the scoobies. Willow feels a little betrayed, but forgives Tara.
Glory, looking for her key, thinks it is Tara. Willow and Tara have a fight over Willow's use of magic, and Tara goes to a fete alone. Glory sucks her brain, and Willow goes after Glory, incredibly angry. Willow opens the dark magic spell book and uses all kinds of things at Glory, saying 'I owe you pain' knives, fire, water, power, until Glory overules her, and Buffy takes Willow home.
In 'The Gift' Willow is urgent to bring Tara's mind back, and she holds Glory's and Tara's heads and Tara's mind is poured back into her.