Our first port of call must be Wikipedia, which says Tanith Lee's "prolific output spans a host of different genres, including adult fantasy, children's fantasy, science fiction, horror, gothic horror, gothic romance, and historical novels. Her style and atmosphere is probably closest to Jack Vance who, similarly, is not tied to a specific genre."
She has certainly written a lot of novels, many of them memorable; her Fantastic Fiction page lists 102 books! "one of the leading fantasy authors writing today", they quite rightly comment. Her own website is tanithlee.com, which seems to be more in the form of a news-blog than the usual multi-page affair.
Personally I've read very little Lee, so I look forward to seeing what other people here have to say!