Willow has changed alot over the seasons. In season 1 she was just that shy little school girl who did everything she was told to do and she was the girl who had a huge crush on her best friend Xander Harris. By the end of season 6 she was a a woman who had just lost the person she loved and she had tryed to end the world - so i think its fair to say she has grown as a person and a witch.
HER RELATIONSHIPS
She has had a relationship with a werewolf called Oz which briefly ended due to her affair with Xander but they got back together only to break up again during season four. Willow then started seeing a Tara a girl that she meet at a Wicca group in collage which hit a rough patch when Oz came back on the scene, but Willow chose Tara and Oz left again their relationship became open and it was solid until they had an arguement (about Willows over-use of magic) during season 5 which lead to Tara being attacked by Glory but Willow saved Tara and They became solid again....
But a happy ending this is not afterall this is Sunnydale...
Willow gets coruppted by magic and Tara threatens her that if she doen't stop using magic they will break up. Willow says she won't but then uses a spell on all of the scoobies and this is the last straw for Tara. She leaves Willow.
ABOUT 5 EPISODES LATER...
Willow and Tara have been talking again, Willows off the magic and she wants Tara back so she goes to the college to talk to Tara but sees her with another girl. Willow leaves and Tara sees but doesn't follow. Next episode....Willow decides to ask Tara for coffee Tara accepts and they talk...and...they kiss!
They are now back together solid...they can't get out of bed but...
The next part is hard to write so i'm gonna do it quickly...
1. Warren shoots Tara.
2. Tara is now dead in the arms of Willow.
3. Willow wants revenge and goes all super powerful.
4. Willow kills Warren. Painfully.
5. Willow trys to end the world.
6. She gets stopped by Xander and his love for her.
Thats it...the whole painful story sad huh? Season 7 yet to come....Watch this space.