LGBT is an abbreviation for Lesbian, Gay, Bisexual and Transgender or Transsexual. It is a convenient and widely used way to group together non-heterosexual forms of sexual interest or involvement, rather than simply say "non-heterosexual".
Sometimes the longer form LGBTQ is used. Q stands for "Questioning", i.e. people who, while not actively engaged in non-heterosexual activity, question their sexual orientation. More recently, the Q has been said to stand for Queer. Another extension is LGBTQ+ with the "+" representing any other "non-heterosexual" inclination.