difference between AAC, matched and purchased

I copied this from about.com, hope it helps:

AAC audio file: This is a standard AAC (Advanced Audio Coding) file. Most likely, you created this file by converting an MP3 or ripping the song from CD using iTunes' built-in AAC encoder.

Matched AAC audio file: This is a standard AAC audio file, except that you downloaded it to your computer or iOS device from your iCloud account, using iTunes Match.

MPEG audio file: This is a standard MP3 file. You may have downloaded it from the web or ripped the song from a CD using iTunes' built-in MP3 encoder.

Protected AAC audio file: A protected AAC file was the default filetype for songs purchased from the iTunes Store prior to the introduction of the DRM-free iTunes Plus format in April 2009. It is noted as being protected because the DRM built into the file prevents it from being copied/shared beyond the iTunes account it's associated with.

Purchased AAC audio file: A purchased AAC file is what a protected AAC file becomes when it's been upgraded to the iTunes Plus format. This file was still purchased at the iTunes Store, but it no longer has the DRM-based copy restrictions. All songs at the iTunes Store sold after April 2009 are in the Purchased AAC audio file format.