When you convert a sound/music file to MP3, you are compressing it to approx 1/10th of its size, hence you are losing quality (but quality is better than 1/10th that of the original AIFF file).
Because you have lost the quality information from the original file in order to save space, you can never recall that quality again from that MP3.
What this means is that if you burn a CD or convert back to a lossless format like AIFF, you are increasing the file size again to 10 times that amount but the quality will remain EXACTLY the same as the MP3. (Therefore, while burning a CD is done practical purposes, there is no advantage whatsoever in converting an MP3 file into an AIFF file to listen to in iTunes as it is the same quality as the MP3 but takes up much more space.)
If you convert the MP3 (or MP3 converted back to AIFF or burnt to a CD) back into MP3 or AAC or WMA, the quality will get worse because you are now compressing the already compressed file a second time.
The same applies to any compressed audio file - like AAC or WMA too.