This marketing page may be useful: http://www.apple.com/itunes/itunes-match/
Regarding iTunes Match, it sounds like all of my music, including non-iTunes purchased music, is uploaded to "a cloud" for storage and later play.
Not every track in your iTunes library is "uploaded." What happens is your iTunes library is "mirrored" in the cloud. Most tracks that you have bought from the iTunes Store will have a status of "purchased," many tracks will be "matched" to tracks in the iTunes Store while the remainder will be actually "uploaded." Also, iTunes Match is not a backup service.