Music File Formats (for Newbies) Options

[Fats Schindee]

Hey everyone, I'm new here and new to computer based recording. I've had iTunes for awhile, but never paid attention to what was going on "behind the scenes" until I recently started using GarageBand. My basic newbie question is this: What is the difference between .mp3, .m4a, and .aif files? All the files uploaded from my CDs on iTunes are .m4a and the ones transferred from GB are .aif. The .aif ones are much bigger file size, and I just found out how to change them to .mp3, which are much smaller. What am I losing/gaining by using the different formats? If there is a primer or tutorial I should check out please lead the way. I tried a search but didn't come up with much. Thanks for any info...