A pansexual is someone who can be sexually attracted to anyone, regardless of gender identity. The term pansexual comes from the Greek term pan, meaning "all." Another term for pansexual is omnisexual but it is not as commonly used. Pansexual doesn't mean that someone is sexually attracted to everyone (just as a heterosexual is not attracted to everyone of the opposite binary gender). As the term refers only to sexual attraction, pansexuals are also capable of having any kind of romantic orientation, including one that does not align with being pansexual (ie. heteroromantic or homoromantic).
Pansexuality is usually considered different from bisexuality as pansexuals can feel attraction to all gender identities rather than just two. However, this distinction is debated and tends to merely fall down to semantics; some people use the terms interchangeably.
Sexual orientation is not linked to gender and anyone of any gender identity can be pansexual.