Apple today released iTunes 10.5.1, a software update that enables iTunes Match, a new music service integrated with iTunes in the Cloud that allows customers to "scan & match" their local music libraries, and store their songs and albums in Apple's cloud. iTunes Match is currently available to U.S. customers only, and it costs $24.99 per year via iTunes subscription; iOS 5.0.1 and iTunes 10.5.1 are required to use iTunes Match.
Unlike other cloud music services, iTunes Match uses a scan technology that enables Apple to match songs in a user's library with the ones the company already has on the iTunes Store, whilst the ones that aren't recognized in the scanning process are directly uploaded to Apple's servers. Because the iTunes Store offers more than 20 million songs, Apple believes most user libraries will be matched in seconds with the catalogue Apple already has, thus avoiding uploading an entire music library, which could take weeks on an average Internet connection. Furthermore, while the songs that aren't matched with Apple's iTunes Store are uploaded "as they are" (the exact file that a user has on a computer), matched songs are automatically upgraded to 256-Kbps AAC DRM-free quality, even if the original copy was of lower quality.
iTunes Match isn't a streaming service like Spotify and Rdio in that it requires users to go through a "scan & match" process before they can start using the service. It is, however, deeply integrated with OS X and iOS 5: on the desktop, iTunes can download and play songs from iTunes Match, and the native Music app on iOS device is capable of downloading any song or album stored in your iCloud account, provided you've enabled iTunes Match in the Settings. iTunes Match doesn't come with a huge catalogue of artists and genres available for streaming right away, but it allows users to scan & match their libraries, and download their songs (matched or not) at any time, on any device, with deep system integration.
At the moment of writing this, iTunes 10.5.1 includes iTunes Match but the service is still mentioned as "beta" within iTunes:
We will have a complete overview of iTunes Match later today on MacStories. iTunes 10.5.1 is available now on Apple's website or through Software Update.