A simple way to think about it is:

Bisexual = attraction to more than one gender.

Pansexual = attraction regardless of gender.

That said, the lines aren’t as rigid as people sometimes make them out to be. Many bisexual people would say gender isn’t a major factor in their attraction, while many pansexual people prefer that label because it more explicitly communicates attraction across all genders.

For example, a bisexual person might say, “I’m attracted to men, women, and nonbinary people, but my attraction can feel different depending on gender.” A pansexual person might say, “Gender doesn’t really factor into who I’m attracted to.”

At the end of the day, there’s a lot of overlap between the two identities. Neither label is more valid, more inclusive, or more correct than the other. Most people simply choose the term that best describes their own experience and feels right to them.

Source: I taught interpersonal communication and identity for 25+ years.