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After a brief delay from the original "end of October" launch window, Apple has finally flipped the switch on iTunes Match today. The $25-a-year subscription service, announced during Apple's WWDC 2011 in June, scans your entire music library and compares it against some 20 million songs in Apple's database. Those that are matched – even if ripped from CDs or acquired illegally – will become available for download onto your Apple devices as high quality 256kbps AAC files, while the rest will be uploaded to your digital locker.
The new iTunes Match service is integrated with the iCloud platform, which taps into the cloud to synchronize files, content, and apps between Apple devices. Although iCloud doesn't technically "stream" music to you on demand, it uses a simultaneous listen and download approach to create the same effect.
You can clear music you've downloaded at any time if you want to free up space and re-download later. The service includes a 25,000 song limit, although iTunes Store purchases don't count towards the total.
To get started with iTunes Match, simply launch iTunes 10.5.1 and look for the new iTunes Match entry in the Store section of the sidebar. Once you subscribe, iTunes Match starts indexing your music right away. You can watch its progress or continue using iTunes while it matches your library.