It is becoming more and more common to see the acronym GLBT extended to GLBTQ. The Q's meaning is a little ambiguous as I've heard it stands for both people who are "Questioning" their orientation or those who choose to label themselves as the more inclusive term "Queer".
It is often used in the phrase "GLBT Community" in order to be more inclusive/politically correct as the term "gay community" or "homosexual community" tend to be strongly associated with the homosexual male population rather than the GLBT community as a whole. Whether there is actually a GLBT community is up for debate, as for the most part, the groups rarely mix together in large amounts. The acronym is most commonly used only in official context: political speeches, papers, announcments, group/club names, etc. Casual speech usually denotes it as the gay community despite the bias.