It works by uploading a database of your iTunes music library to Apple's servers. It then compares that list with the 20 million tracks offered in the iTunes Store; anything already available from Apple becomes instantly downloadable to your iOS devices and other computers running iTunes in 256-kbps AAC format--even if your songs are low-quality MP3 files ripped from your CDs years ago.
For music that the service can't match up with iTunes Store content, iTunes Match lets you upload the actual tracks to the iCloud servers. The end result is that all of your music--or at least up to 25,000 non-purchased tracks--is available for you to listen to and download to any of your iOS devices or computers running iTunes.
We'll have many more details on how it works when iTunes Match is available later in the month.
Jonathan Seff is executive editor of Macworld.com.