The tool will allow you to sync music, photos, personal videos, TV shows, podcasts and movies. The interface is decidedly iTune-ish and allows you to sync content in several different ways. For example, if you select the music section, you can see your iTunes playlists, artists and genres. Just select the items you want and press the Sync button.
To keep the clutter down, any DRM locked content isn't shown in the interface at all as that couldn't be played on the phone anyway. Anyone with a lot of DRM content from the iTunes store that wants to sync with a Windows Phone should consider paying the minor fee per song to convert to unlocked MP3 files. Even if you have no desire to sync, you should do that anyway. Chances that those locked files will continue to work in five, ten or fifteen years from now are pretty slim.
As I noted earlier, this isn't a full Zune desktop client like on Windows machines. It doesn't maintain its own library. It also doesn't allow you to buy or download any music. This means the Zune Pass is out. The Zune Pass is a $14.95 fee that allows you to synchronize just about any song in the Zune's online store as long as you continue to pay the monthly fee. On top of that, you get to pick ten songs each month to keep in an unlocked MP3 format.
You can download the client from Microsoft's site. Note the tool is still in beta so there are likely to be glitches and periodic software updates. It also requires OSX 10.5 or higher. If you want to see it in action before downloading, The Bleeding Edge has created a nice walkthrough video.