There are many different definitions for the term transgenderist. The one I'm giving here is the one as it's used often by Dutch transgendered people: a person who is somewhere inbetween transvestite and transsexual. This could mean someone who is physically male but chooses to live life in female clothes, or even someone who has decided to take hormonal replacement therapy, but not to go for the complete sex reassignment surgery.
Transgenderists come in many different flavours.