Willow first met Tara through the on-campus Wicca group. Tara, who was a real witch as opposed to the other "wanna-blessed-be's", seeked Willow out and the two began to do spells together.
With Oz out of the picture, trying to find himself, Tara and Willow became close.. very close. Eventually Willow introduced Tara to her other friends, though she held off for quite some time before doing so.
At a party, when Willow put her hand on Tara's thigh in a loving gesture, Tara flipped out and yelled, "Don't touch me! Th-that's dirty!"
Needless to say, the twosome of cuteness solved things. Tara consulted Willow before she got a cat, claiming that she wanted her room to be "Willow-friendly".
Re-enter Oz. When finally faced with a choice, possibly the hardest one of her life, Willow told Oz that she would always love him,, but it just wasn't meant to be right now. She returned to Tara, who told her, "You have to be with the person you love." To which Willow replied, "I am."
Aww!
Coming out to Buffy was a difficult thing for Willow to go through, and Buffy initially freaked about it. She was uncomfortable because she hadn't told anyone about her and Faith, so homosexuality was a touchy subject for her.
Tara's family does not know about her and Willow, and probably never will. They were crazy enough about her choice to live the Wiccan lifestyle. Who knows how they would react to her choosing an alternate romantic lifestyle as well?
I'm not aware of any other pages that have a detailed explanation of Willow and Tara's relationship, but if you find or have one you think is worthy, please send it to me and I'll check it out. Thanks!