I think my title shows my confusion/???.

I'd like to know what's the difference between, say, B4 and A4 paper, what they even are, what you use...and so on. Basically, everything you can tell me. >3> Thankees.

They are paper sizes, B4 is a little bigger
B4= 250 × 353
A4= 210 × 297

The A and B sizes were designed in such way so that you can fold the pages in half to get to the new size. So let's say you have a A1 poster, you fold it in half and you got a A2 poster. This folded in half will give you A3, which is a small poster that's offered in magazines... and magazines/euro comics are offered in A4 which is that poster folded in half And half of that is A5 (which is used for school notebooks here), half of that A6 (pocket notebooks) and so on...

The thing is all these are proportional to each other... the same with B sizes, which are placed between the A sizes. You can't get them by folding A sizes though but again, B5 is half of B4 which is half of B3 etc, and they're at the same scale anyway.

And yes, B4 is larger than A4