Diane was a waitress at Cheers.
After a childhood of privilidge, Diane became a career student of sorts, spending most of her early adulthood in various studies. Eventually she found herself as a teaching assistant to Sumner Sloan. The two began a relationship, and eventually became engaged. They stopped off at a bar before heading off on vacation where they were to get married, but Sumner wanted to get a special wedding ring from his ex-wife. He left to get the ring, and asked the bartender to keep an eye on his fiancée.
The bar, of course, was Cheers, and the bartender Sam Malone. In his short visit with his ex-wife, Sumner had rekindled their relationship, and not only was Diane dumped, Sumner's ex-wife wouldn't allow him to keep a former lover as a TA, so Diane was jobless as well. Out of pity, Sam offered her a waitressing job, and Diane reluctantly accepted.
She was completely out of place, an intellectual amongst a pack of beer-swilling jocks and couch potatoes, but while she sought to bring culture to the unwashed masses, she occasionally revealed her deep desire to be accepted as one of the gang.
Diane and Sam rather quickly developed a love-hate relationship, and on occasion, sparks flew. They started up a relationship which ended rather quickly. Though they had strong feelings for each other, their interests were so divergent (his three stooges to her three tenors) a relationship was difficult. She later became involved with Frasier Crane, a more logical match for her intellect. The two were nearly married in Europe, but Diane left him at the altar, feeling confused about her feelings for Sam. She and Sam rekindled their romance, and after a tremendous struggle, they were eventually engaged to be married.
Her plan to be married was put on hold, though, when a once in a lifetime opportunity came along. A publisher was interested in one of Diane's old school papers, so she set off on a retreat to prepare the work for publishing. She assured Sam it would only take six months. Sam was convinced she was leaving for good, and his instincts proved correct.
It wasn't until the series finale that we learned what happened to Diane. Diane's manuscript grew to an unmanageable length, and she was afraid to return to Cheers until she was a success. It was only when she "trimmed a few thousand pages" and turned it into a screenplay that she had anything to brag about.