That's a great question. Some of these terms certainly overlap with each other and overlap with transgender, but not always. For people who view gender as nonbinary (more than just either man or woman), these terms are often the most common alternatives to man and woman. I wanted to be as inclusive as possible--giving people as many options as possible to describe their gender.
The Washington Post put together a nice article defining these terms (in addition to others):
Gender Questioning: A person who is unsure of their gender identification, or who is experimenting with different genders.
Gender fluid: A person whose gender identification shifts.
Gender Nonconforming/Variant: A person who doesn’t conform to social expectations of gender expression.
Genderqueer: A person whose gender identity is neither man nor woman, is between or beyond genders, or is some combination of genders. “Queer” is an old word — dating back to the early 16th century — and its early application to LGBT people was often derogatory, as in its original meaning: “Differing in some odd way from what is usual or normal.” In the past 20 years, however, the term has been reclaimed by advocates — though Merriam-Webster cautions that some academics still find it offensive.
I also really like UC Berkely's list of definitions: http://geneq.berkeley.edu/lgbt_resources_definiton_of_terms