What is a compressed Wav file and how does it differ in quality from a regular wav file or from a MP3 file?

There are two groups of audio data, compressed and uncompressed data. The compressed group could also be splitted to lossless and lossy compression. Lossless compression is like having uncompressed data, only that the file is only around 70% of the uncompressed size (Comparable to compression with WinZIP). To the group of lossy compression also belongs MP3, it is not possible to recreate the original audio file 100%, there are frequencies missing, etc. Now, MP3 is ONE lossy compression format, there are others like AAC, MP2, TAC, etc. The nice side on WAVs is that it could be wrapped on any compression for which an audio codec exists in Windows. So if you own the Fraunhofer Codec (or the LAME/Blade/Gogo DLL) you could produce MP3-WAVs. These are standard MP3 files, but having a small header preceeding the actual data telling the player what codec to use for playing/decompression. So these files could be played with any media player or any other sound tool. As the header does not matter on MP3 compression, you could even rename the MP3-WAVs from .wav to .mp3 without loosing playability by a MP3 player.