Well we can take the use of queer in many different ways.
One can look at it as an area of study. For example, in looking at the majors at the local state university (Rutgers) there are various different types of area studies- African and African American, American, Central European, East Asian, Gender, Women's- you get the point. Queer studies is just one of those.
One can also understand that "queer" is a taking over by those with an oppressed sexual orientation. In other words, straight folks use queer as term of insult so we will take it and use it as a source of pride.
However, this misrepresents some current understandings of queerness and how it is used in various theoretical frameworks. To go back to the point I made at the beginnng, Queer refers to a form of acamdemic study. Actually I would say it started off as a subset of women's studies that eventually grew into it's own area of academic interesy.
In looking at it this way, gender studies would try to examine how the influence of gender determines how people understand and interact with the world. Queer studies would look at how people who are not included within the traditional gender norms understand the world.
This brings up a number of different and interesting questions: would all glbtq (add more letters) people be queer? Can a straight person be queer? Is there such a thing as a queer understanding of the world? What does this have to do with a political, economic, socail ordering of life?
Therefore, queerness, as understood in this framework, is a way of understanding the world through one's sexuality that is outside of the traditional gender norms. By giving this definition of queerness, it does not preclude the obvious point that people understand the world in different ways in addition to thier sexuality which also influences thier world view. In addition, the word "queer" is then used as a title for all those who somehow identifiy outside of these norms irregardless of who they sleep with. Therefore, straight, gay, lesbian, bi, trans- anybody- can define themselves as queer.