Mosaics are white with another color, usually grey, splotched over their coat. Not every mosaic has the same pattern. Most white chins are mosaics so if you look through the forums and see a white chin, it's probably a mosaic you're looking at.
A white chinchilla that isn't a PW (pink ears, red eyes) basically, is a mosaic. Then people break them down into white/ebs, white/violets, white/sapphires so that you know what is in their lines.
Panda mosaics, upside down mosaics, purple polka dotted mosaics - they are just mosaics.
I have two mosaics: Minnie and Trousers. Their coat patterns are VERY different. Minnie has dark ears and the base of her tail is dark, and Trousers has dark ears and looks like she is wearing dark gray pants (pics of both in chin chat section). The cool thing about mosaics is that no two are exactly alike.
Are you curious about other colors too? I remember someone posting a thread about the different colors with lots of pictures...