What's the difference between Pansexual and Bisexual?

I wish there was a better way of asking that...

Anyway, I hear people referencing these two a lot, and I was just wondering when someone stopped being Bi and became Pan etc.

I know ome people get touchy about this kind of question, but honestly I mean no offence to anyone. I just want to know so I don't look like and idiot. :)
Thanks in advance. ♥