In general, if someone describes themselves as bisexual it usually means they are attracted to both people of the same gender and people of a different gender.
Pansexual means pretty much the same thing – that a person is attracted to people of any gender. The term more explicitly acknowledges the existence of more than just two genders, and is preferred by some for this reason.
The meaning of these two terms is very similar, but is open to debate, and the truth is that one person saying they are (for example) bisexual may mean something subtly different to another. If in doubt, ask!
See sexuality for more on this.