I always thought "queer" was just slang/an alternate term for "gay"... what's the difference?
"Queer" is often used by people who feel they don't fit into a clearly-defined label (like lesbian, male or female, gay, etc). It can be used by people to describe their sexuality or their gender identity. For instance, "gender queer" refers to an individual who feels they don't fit into either a male or female identity, but somewhere in between or totally seperate (or in some cases, they feel they alternate between male and female).
Queer is also what some people refer to as a "reclaimed" word - it was originally derogative, but was reclaimed by the LGBTQ community as a way of expressing pride. So for some LGBTQ folks, identifying as "queer" instead of lesbian, bi, or gay etc is a way of expressing pride and individuality. Some folks just hate labels, and use queer as a way of identifying as different, without putting themselves into a specific box or label.
It's sort of an umbrella term in some ways, as people who are bi, lesbian, gay, etc can all identify with the term queer.
But there are so many identities and vastly different POV's within the LGBTQ community, it can mean different things to different people or groups.
Hope that clears it up a bit!