Sexuality encompasses a broad spectrum of behaviors and definitions. Sexual orientation refers to sexual preference, or to whom you are most attracted, with heterosexual (primarily attracted to the opposite gender) and homosexual (attracted to the same sex) being two ends of the attraction spectrum, with bisexuality somewhere in the middle. There are also those who choose to define themselves further as pansexual (attraction to people regardless of their gender identity or biological sex.)
From Wikipedia: LGBT (or GLBT) is an abbreviation used as a collective term to refer to Lesbian, Gay, Bisexual, and Transgender people. It is an adaptation of the abbreviation LGB. While still controversial, it is considered less controversial than queer and is more comprehensive than homosexual or simply gay. The acronym GLBT is sometimes used in the United States and commonly in Australia, but to a lesser extent elsewhere.
As of 2005, LGBT has become so mainstream that it has been adopted by the majority of lesbian, gay, bisexual, and transgender community centers and the LGBT press in most English-speaking countries.