Well there is a difference, sort of, between bisexual and pansexual. With bisexuality, you are going on the idea that gender is binary. Pansexuals take that binary and throw it out the window. Gender is more than just girl and boy. There is a range of genders and sexualities and pansexuals are attracted to people rather than a gender or sexuality.
Take me for example. I am biologically female, though if someone were to ask me what gender I am, I'd probably not have an answer for you. I'm sort of something inbetween. The term pansexual refers to people who take this kind of gender identity (and other gender identities) into account and is attracted to person regardless of gender expression or their sexual orientation. I am personally attracted to people, not what they have downstairs or how they identify.