There are two types of mainstream audio. There is mp3, the most popular and most used, with a frequency of 320kbps. It is the opposite of FLAC. While FLAC is lossless, meaning there is no loss in quality. But FLAC is very large (15-25 MB per song) and the format is not compatible with 95% of portable and digital audio players. However, mp3 is lossy, meaning loss in the original quality. But it is the most compatible and most popular and is very small (1 MB/2 Minutes of Audio). But there is also m4a, Advanced Audio Coding (aac). These files tend to have larger sizes (6 MB/4 Mins of Audio) but the reason they are larger is because they are encoded differently. The audio file size is still reasonable and the quality is maximum. The format is mostly used on Apple iTunes. Apple even has it's own general media formats: m4a for audio, m4v for video. All music videos are encoded in the m4v format. M4v is a video format encoded in natural better quality than mp4.
So, what is untouched, iPlus, iTunes Plus, all that jazz? Well here's the answer plain 'n' simple. If a file is untouched, it means that it was purchased directly from iTunes. It has not been re-coded or processed, just straight from the source. It hasn't been touched. iTunes Plus is just a fancy name for it. Now, however there is iRip, meaning an audio file ripped from a CD-ROM is a m4a format. These m4a's are high in quality, in fact they are the highest quality, but they are not untouched. They have been ripped from a CD and processed, not from iTunes. These files are called iRip. There is iRare, meaning that either a purchased file that is no longer on iTunes, or an iRip of an album not on iTunes. There is iBundle, meaning files put together in an archive that are not put together in it's original package.
Well, hope that answers your question. :) Email me at email@example.com for any other questions. Thanks.