Bisexual and pansexual attraction overlap a lot, but they emphasize different things.
Bisexual attraction acknowledges gender as one dimension of who you're drawn to, without being limited to just one gender. See the full Bisexual page.
Pansexual attraction is gender-blind - gender isn't a factor in who someone is attracted to at all. See the full Pansexual page.
Both fall under the Multisexual / M-spec umbrella alongside Omnisexual, Polysexual, and Spectrasexual. Many bi and pan people describe their own experience the same way - the two labels exist because different language fits different people better, not because there's a hard boundary between them. Choosing one over the other is a matter of self-identification, not a test either community enforces.