Amazon MP3 is an online music store that sells DRM-free songs and albums. It was launched in September 2007.
iTunes is Apple's online music store that was launched in April 2003. Songs are DRM-free.
Amazon MP3 offers DRM-free downloads of over 2 million songs from 180,000 artists and 20,000 labels. In comparison, Apple says the iTunes Store now contains over 6 million songs.
edit File format and Sound Quality
AAC was devised as the successor to the MP3 standard. It achieves better sound quality than the MP3 format when compared at the same bit-rate .
Both Apple and Amazon sell songs without DRM and in open formats, AAC and MP3 respectively. Any music player will play these files.
As of 2011, The Amazon MP3 service is available for consumers in the United States, United Kingdom, Germany, France, Austria, Switzerland, and Japan.
iTunes is available in France, Germany, UK, Austria, Belgium, Finland, Greece, Italy, Luxembourg, Holland, Spain, Portugal, Canada, Denmark, Switzerland, Sweden, Japan, Norway, Australia and New Zealand. See also: Map of iTunes availability
Between the two services, pricing is more or less the same. Most songs on Amazon MP3 range in from $0.25 to $1.29, with albums priced from as low as $4.99 to as high as $17.49, depending on artist, length of album and whether or not the album is "Deluxe" or "Special". On iTunes, the pricing is approximately the same, though a quick comparison shows that some select songs and albums are more expensive on iTunes, while others are more expensive on Amazon.
Ultimately there's little to no difference.
edit Ease of use with iPod/iPhone
Songs sold by both music stores are compatible with the iPod and iPhone via iTunes, though due to the nature of the iTunes Store's integration within iTunes, it is arguably an easier and more seamless experience.
If you have iTunes installed on your computer, Amazon downloads mp3 format to a location of your choice and also downloads to your iTunes library. If iTunes is not found on a Windows computer, it copies to Windows Media Player, instead.