Genderqueer is an umbrella term that people use to describe a gender identity that doesn't fit in the binary of male and female identities. It's a relatively new term (introduced sometime in the 1990s), and people use it in different ways. Probably the most common element of the term is that it's used by people who don't identify as entirely male or female.
Some people use genderqueer as synonymous with gender non-conforming or gender variant. Others who prefer this term like it because it describe an active belief that binaries (either/or choices) are restrictive for everyone when it comes to gender. While gender non-conforming may describe how someone's identity or presentation conflicts with other people's expectations - without rejecting that those expectations work for others - genderqueer may be a more active suggestion that expectations themselves are a problem. Just as queer is a term that, for many, has particular political meaning, so too can genderqueer.
For some people, being genderqueer is about their gender identity, and says nothing about their sexual orientation. For others, being genderqueer means that they reject binaries of both gender and sexual orientation.