Originally Posted by DaveF
What does it do besides backup music? Is it to be able to swap out music sitting at Starbucks?
When you buy a song on iTunes, iCloud has *a reference to* the One True authoritative copy. You can download it to up to ten devices, up to five of which can be computers. (It might ID-stamp the download on the fly to contain your Apple ID.)
When you Match, it puts a reference to the iTunes authoritative copy in a separate bucket. (It's the same single copy of the song that people that bought the song use.) Just like purchased music, you can download that whenever; and repeatedly, if you removed it to make room for something else. When/if your Match subscription expires, that bucket goes away. You still have all copies of that song sitting on your devices -- since they're DRM-free copies, they can't do anything about that anyway.
If the song doesn't actually match, then it will upload your copy and hold it for you, counting against the 5GB limit. (You can buy more storage.) There's also a total limit of 25,000 songs, matched or uploaded.
That's my understanding of how it works. I may be wrong; and they could change it.