Sexual orientation describes the gender of the people you are attracted to. Your sexual orientation can be gay, lesbian, bisexual, or straight.
You will probably be happy to hear that although I don't think it's actually a sexual orientation, there are people who are specifically attracted to trans people. For example, I have met quite a few women who like to date women and trans men. Rather than limit themselves by using the term "lesbian," these women prefer the term "queer" because they find it more inclusive of who they are attracted to.