Well, bi technically means boy, girl, etc. But boy and girl aren't the only genders out there. I have a good friend, for instance, who identifies as demi-boy, meaning they have an identity somewhere in between boy and girl, but it's still closer to the more masculine side of their personality. Bisexual/biromantic means you can be attracted to a few genders, not just boys and girls. However, pan means you're attracted to everyone. There's almost no difference, though, except the flags and which one you like better, so I tend to go with the pan flag because it's brighter.
I'll show you guys!
http://data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAQMAAADCCAMAAAB6zFdcAAAAGFBMVEX/IYwhsf//2AD/Fo7/b2//3gD/2gCBvstQQlp1AAAA20lEQVR4nO3QSQGAMBAAsS1X/TuGUcEnkZAZAAAAAAAAAAAAAAAAAAAA4FcnczE3c+DAQRw4iAMHceAgDhzEgYM4cBAHDuLAQRw4iAMHceAgDhzEgYM4cBAHDuLAQRw4iAMHceAgDhzEgYM4cBAHDuLAQRw4iAMHceAgDhzEgYM4+A4eZjMLBw7iwEEcOIgDB3HgIA4cxIGDOHAQBw7iwEEcOIgDB3HgIA4cxIGDOHAQBw7iwEEcOIgDB3HgIA4cxIGDOHAQBw7iwEEcOIgDB3HgIA4cxIGDOFjrBZSs14BB7sCFAAAAAElFTkSuQmCC This is the pan flag!
http://data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAASIAAACuCAMAAAClZfCTAAAARVBMVEUAOKjXAnBzT5bXAGsALKa8IXwwP6PZAG/Smq+aocFuR5Owp7vTmK7ZAGq9v8qXn8AAH6TYAGXInbIjNaGgo7+7DXfQvcR+HeiyAAABf0lEQVR4nO3Qi0HCQABAsVNQCxTLf/9RxQXuLZCMkPFaFybW11ivOyau61h2H0zsFkVBUVKUFCVFSVFSlBQlRUlRUpQUJUVJUVKUFCVFSVFSlBQlRUlRUpQUJUVJUVKUFCVFSVFSlBQlRUlRUpQUJUVJUVKUFCVFSVFSlBQlRUlRUpQUJUVJUVKUFCVFSVFSlBQlRUlRUpQUJUVJUVKUFCVF6b/o/MPEeRm35xcTz9s4HT+ZOJ4UBUVJUVKUFCVFSVFSlBQlRUlRUpQUJUVJUVKUFCVFSVFSlBQlRUlRUpQUJUVJUVKUFCVFSVFSlBSld9Hj/s3E/TEO+8HE/qAoKEqKkqKkKClKipKipCgpSoqSoqQoKUqKkqKkKClKipKipCgpSoqSoqQoKUqKkqKkKClKipKipCgpSoqSoqQoKUqKkqKkKClKipKipCgpSoqSoqQoKUqKkqKkKClKipKipCgpSoqSoqQoKUqKkqKkKClK76Ltsmfiso3f7cDE9vsHrxzYa42TOXMAAAAASUVORK5CYII= And this is the bi flag!
I identify with pan myself, but both flags are pretty and I really like them. It's a little hard to understand difference in genders, but let's try thinking about it this way.
There are as many genders out there as people in the world. Everyone's gender is a part of their personality and self-expression. Let's take Lucy as an example. She's a girl, clearly, and while she can be somewhat "girly," she can also be pretty tough and exhibit some masculine features, such as never wearing skirts or dresses. When she goes to a formal setting, you would usually find her in a suit and tie because she finds dresses and skirts uncomfortable. Thus her gender expression and overall gender in general is different from just "girl". It's a social practice no one needs.