How does iCloud compare with Dropbox and SugarSync?
Unlike Dropbox and SugarSync, which are designed to let you sync any file or folder on your computer through their servies, iCloud is focused on integration with apps. So it doesn't matter where you save that Pages document on your Mac; it'll be synced with iCloud merely by virtue of being a Pages document. On the other hand, you won't be able to--at least from what we know so far--use iCloud to sync a document created in an application that doesn't offer iCloud support.
What about photos?
iCloud includes a feature called Photo Stream, which auto-imports any new pictures taken on an iOS device or added to iPhoto and stores them for 30 days in the cloud. You can view those photos on (and in some cases download them to) your other iOS devices, computers, and even your Apple TV. Because of their size, photo syncing is limited to 1000 pictures on iOS devices, but is unlimited on computers. (Although you obviously can't run iPhoto on a Windows PC, Apple says Photo Stream will work with those computers as well--you'll simply choose a folder on your hard drive that you want to use as your "photo library".)
Only 1000 photos? What about all my other photos?
iCloud stores and syncs your most-recent 1000 photos. But that doesn't mean you lose older photos. Apple said on Monday that any photos in iPhoto (or, on a Windows PC, to another folder) will be permanently stored on your computer.
What kind of music features does iCloud have?
iTunes in the Cloud offers manual downloading of all your previously purchased iTunes Store music, as well as automatic downloads of all new purchases, to any computer or iOS device authorized for your iTunes account. If you've purchased tracks from the iTunes Store in the old 128-kbps protected AAC format and re-download them, they'll be delivered in that same format--they won't be offered to you as unprotected 256-kbps AAC files (for that you'll have to pay 30 cents a track to upgrade them through iTunes). If tracks that you've purchased are no longer available from the iTunes Store, you won't be able to re-download them.
Currently, streaming music from iCloud is not supported, which means in order to listen to a song or album, you'll need to download it to the device or computer on which you want to listen to the music. For iOS devices especially, storage will be a limiting factor. There's also a new feature called iTunes Match (as we'll explain in the very next item).
What about music that isn't from the iTunes Store (stuff I ripped from CDs myself, say)?