A music download is the transferral of a song from an Internet-facing computer or website to a user's local computer. This term encompasses both legal downloads and downloads of copyright material without permission or payment.
Popular examples of online music stores that sell digital singles and albums include the iTunes Store, Napster, Zune Marketplace, Amazon MP3, TuneTribe, Kazaa and eMusic. Paid downloads are sometimes encoded with Digital Rights Management that restricts making extra copies of the music or playing purchased songs on certain digital audio players. They are almost always compressed using a lossy codec (usually MPEG-1 Layer 3 or Windows Media), reducing file size and therefore bandwidth requirements. However, this may cause an apparent loss in quality to a listener when compared to a CD, and cause compatibility issues with certain software and devices. Uncompressed files and losslessly compressed files are available at some sites.
As of 2006, digital music sales are estimated to have reached a trade value of approximately US$2 billion,[1] with tracks available through 500 online services located in 40 countries, representing around 10 percent of the total global music market. Around the world in 2006, an estimated five billion songs, equating to 38,000 years in music, were swapped on peer-to-peer websites, while 509 million were purchased online.
WikiPedia
