Basically, Transgender is the opposite of unigender . I'll not attempt to match Nancy Nangeroni's excellent discussion on transgender here. I'll only contribute some of my own particular thoughts and predelictions. The perspectives presented here are my own only.
Some people assign different labels to different nuances of transgender.
I prefer to avoid labels, and to think of transgender as a continuum, with crossdressers on one end and transsexuals on the other. Transgender is an issue of gender identity. Gender is not the same as sex. Of the three, only gender identity is entirely subjective. The only way to tell if a person is a man or a woman is to ask that person.