The main difference is how many genders is a person attracted to. Bisexuals, at least in theory, only feel attraction to males or females, and not to any non-binary gender (such as: Agender, transgender, genderfluid.) However, this is just in theory, and in fact some bisexuals can feel attraction to these other genders as well. Pansexuals can feel attraction to these other genders.
In short, it is very similar bisexuality, though people who are pansexual can feel attraction to more genders than those given by the gender binary (male and female.)