Most people are either male or female, including most transsexual people. However, not everyone neatly fits into the "man" or "woman" or "male" or "female" categories. Some people, for instance, have a gender that is distinct from either male or female or a gender that combines aspects of both. Some individuals do not identify as either gender. Over time, some people's genders can change.
Nonbinary (often spelt with a hyphen, as in "non-binary") is one of the most popular phrases used to describe people whose gender is neither male nor female. Other phrases include agender, bigender, genderfluid, and many others. All of these phrases refer to a gender experience, even though they don't all mean the same thing.