What is the difference between non-polar and bi-polar capacitors?
The terms can be used interchangeably to mean the same thing, or to note a difference in design.
A capacitor of ceramic construction is something that is truly non-polar by design. You can still call it bi-polar, but non-polar would be more appropriate. It it inherently non-polar, no tricks or whatever.
An aluminum electrolytic capacitor marked NP or BP uses the circuit trick of connecting 2 of them in series to achieve lack of polarization. That's why such a cap might be twice as big as the regular polarized version - they did the trick for you, connected 2 of them internally in series.
Bi-polar might be used more when it comes to this hack to achieve lack of polarization with electrolytic aluminum capacitors by connecting 2 of them in series either by internal design or you doing it like in a speaker let's say.
Other than that, yeah, you'll see the terms used interchangeably...