Amazon's week-old music download service is just the latest challenge to the 4 1/2-year-old heavyweight known as Apple's iTunes Music Store, which has sold more than 3 billion songs. Most music bought from the iTunes store will play only on Apple's iPod devices, though last May, Apple launched iTunes Plus to sell certain songs without that restrictive, digital rights management coding for $1.29 each.

iTunes vs. Amazon: The Devil's in the DRM Details