File Extensions

Hi - I've been looking at LIMEWIRE, and I noticed that some of the files which people are putting up for sharing have extensions such as ".SIT & .DMG".

Would somebody please tell me what these are and how they should be opened or burned to CD should one wish to do that. Also, would it be possible to download them onto a p.c. and burn them to cd for use on a mac.

Cheers
Rick