The history of the word queer is both generational and geographical – it used to be a common insult hurled at LGBT people from the 1960s to the 1990s. In very limited ways is still an insult to this day depending on where you are and who you are talking to. The word has been reclaimed by a younger generation of LGBT people and it is now seen as a term of wide identification. Beware of the split though, some people see queer as a positive term, others (especially the older LGBT generations) are still offended by it. Understand and respect your audience.
Queer is considered an umbrella catch-all term for LGBT labels. If you don’t want to be labelled or don’t fit into a specific label then queer is a good term to use.
Anyone who wants to identify as queer can use the term, it is a fluid label that many different types of people can use. A great explination can been below by the wonderful Erika Moen: