Transgender is a term which refers to a person who has a gender that does not align with their sex.
This is an umbrella term which encompasses terms such as non-gender, androgynes, transexual, etc.
Because the term is so broad, it is often used in reference to a group as opposed to a specific individual.
Jake: “You should come along to our Transgender Support group, Liam.”
Liam: “Are you sure? I’m not Transgender... You know I’m non:gender, right?”
Jake: “Yeah, Non:gender falls under the term Transgender. Plus you can come as a ally even if you don't identify yourself as "trans".”