Hi Lorraine,
The difference is that AIFF is not compressed, but AAC is. An AAC file compressed at 128kbps will take up about 1MB per minute of stereo audio. AIFF, however, will take up about ten times that amount, or 10 MB per minute of stereo audio.
The tradeoff for file size is audio quality. If you import AIFFs from CDs, they'll sound exactly like the CD, but they'll also be really big. AACs take less space, but don't sound as good as the original CD because they use a "lossy" compression scheme.
Apple has another compression format called "Apple Lossless". It makes files that are about half the size of the AIFFs, but they should sound the same as the AIFFs.
Note that because the AACs are "lossy", you can't convert an AAC to an AIFF and get back the quality you lost making the AAC.
charlie