The term LGBT has been used since the early 90's as many felt the term "gay community" didn't accurately represent all those to whom it referred.
LGBT stands for:
Lesbian - Women who are sexually and emotionally attracted to other women
Gay - Someone who is sexually and emotionally attracted to people of the same sex. This is most often used to refer to men who identify as gay. Many women also identify as gay.
Bisexual - People who can be emotionally and physically attracted to people of either sex
Transgender - people who don't conform to the traditional division of male and female, e.g. people who wish to live as a member of the opposite sex.