What does it mean exactly if someones asks me for a .PNG file which is 24bit?
It's actually quite simple. You have few types of PNG files.
1) PNG-8, which stands for 8 bit, supports up to 256 colour indexed palette. Which works very similar to GIF files.
2) PNG-24 works very similar to PNG-8, it however supports to over 16 million colours (2^24 = 16,777,216 to be exact).
3) PNG-32 which is pretty much PNG-24 + Full alpha transparency