LGBT is a term used to mean Lesbian, Gay, Bisexual or Trans (although the "T" word is sometimes given as Transgender or Transgendered). It is sometimes shown as LGB&T, the ampersand indicating that trans people are in many ways different from LGB people, and have different needs and different experiences.
In the United States, the term GLBT is generally used instead.
The Welsh equivalent is LHDT (lesbiaidd, hoyw, deurywiol a thrawsryweddol).
Sometimes the term is extended as LGBTQ, the Q standing for Questioning (especially when referring to younger people who may be unsure about their sexual orientation or gender identity) or to LGBTQQI (... Queer, Questioning, Intersex; also sometimes written LGBTIQQ) or even LGBTQIA or LGBTQQIA (A for "Allies").