In terms of quality aac 256 vs CD?

Answer 1

Short answer: For most people, it's virtually impossible to hear the difference between a 256 kbps AAC audio file (which is currently the format available from iTunes) and a CD. If you're using very high quality sound equiptment, you may be able to hear a difference depending on how trained your ears are. Even then, it won't be very noticable. Still, AAC is a "lossy" format, so audio data is lost when you rip from a CD to AAC, even if your ears can't tell the difference. If you convert your AAC files to MP3 or any other "lossy" format, you will loose more sound quality with each conversion. That's partly why some audiophiles prefer to rip CDs to a lossless format such as FLAC and .wav files. These files take up much more space, but you won't loose any audio data.