Genderqueer is a non-normative gender identity and gender expression that doesn't fall under the binary of man and woman. A person that identifies as genderqueer may either be using the label as an unbrella term for nonbinary expression. They may also be using genderqueer as an identity as to not be more specific about the nature of their gender.
Genderqueer as an umbrella term, which predates nonbinary by several years, means non-normative or queer gender identity while nonbinary means lying outside of the gender binary, though they both have a very similar scope. Genderqueer may identify as having overlap or blurred lines between genders, may be without a gender, may have two or more genders, may move between genders or have a fluctulating identity, may have a third gender that does or doesn't have a name, or anything else outside of what is considered binary.
However, many nonbinary gender identities reject the term genderqueer as their own because of it's more specific association of being a part of the queer community when they do not consider themselves a part.