Instead of being a stand-alone web destination or separate mobile app though, it will be be part of the iTunes app. That makes it available for an a desktop computer. If you're outside of the US, licensing issues will take time to resolve, look for a release date in the future.
There will be iTunes exclusives too, so expect pre-release album streams to compete with NPR Music, Pitchfork Advance, Rdio and whoever else offers album previews. This shouldn't replace the others, just add one more competitor. Pitchfork and NPR Music cater to niche crowds, and while Apple does that too, it's a much more mainstream audience that's likely to snag previews from major stars.
The iTunes Festival that's happening right now might get a little play this year but should be a full feature in 2014, and there's talk of a "live listen" patent from Apple which might involve live streaming capabilities. Think possibilities: live streaming of concerts, on location events, or even from dedicated DJ channels. This could be used for more than just music.
For iCloud people, what you're paying will already give ad-free iTunes Radio with your service fee for iTunes Match.
If you're an Apple die-hard with more than one device, iTunes Radio should sync across all of your devices with just your Apple ID. This means if you have an iPhone, but also use iTunes Radio on a PC (like a work computer) you should see a sync between your playlists and iTunes Radio stations. iPad streaming and iPhone streaming should be seamless.
If you want to find all of the reasons why Apple would want to cannibalize their iTunes downloads, check out some of my reasons here and here. In short: If Apple doesn't, Spotify and the others will. iTunes Radio can also help drive download sales if they get the user experience right with a one click to buy. Nobody is in a better position to tap into that potential than Apple and their ecosystem, so we'll have to see how that plays out in the next couple of months.