For music it works like this: Any music you've purchased through iTunes is instantly available for streaming -- you don't have to upload it first, as you do with Amazon and Google's cloud systems. The iTunes software was just released with the updates that show all the music you've purchased through iTunes and give you a way to download it again. So it's supposed to be really easy (obviously, we haven't tried it yet).
What about all the music you have on your computer that you didn't buy on iTunes? Well, for a $24.99 annual fee, Apple also offers iTunes Match, which scans your music library and allows you to stream the songs you already own -- but didn't buy on iTunes -- from the cloud at 256 Kbps. iTunes has 18 million songs up there, so it may have most of your collection. But if not, the service will also upload your files to the cloud -- so, in theory anyway, you could access your entire music library on any Internet-connected device.
We're gonna go ahead and say this is pretty cool. In lieu of the 500GB iPod classic we've been wanting since, like, forever, we now have a way to get to all our music on the go -- as long as we can connect to the Internet, that is. Time will tell about the bugs and messy details -- and whether people actually use this -- but there doesn't seem to be much question that Apple today unveiled the best cloud music offering so far.