We're getting a better and better idea of what iCloud's going to look like—music streaming, maybe something Time Capsule-y. But not the whole picture. The typically accurate John Gruber suggests a handy way to conceptualise iCloud:
Don't think of iCloud as the new MobileMe; think of iCloud as the new iTunes.
Albeit it's what he calls "fourth-hand information," here's what he means (to quote him liberally, again):
With iPhones, iPods, and iPads, the central store for almost all data stored on the devices is iTunes running on your Mac or PC. With iCloud, that should shift to the cloud. iTunes, the desktop app, currently syncs the following things with iOS devices: audio, movies and TV shows, iBooks e-books, App Store apps, contacts, calendars, bookmarks, notes, and any sort of files shared between iOS apps. All of these things would be better served syncing over-the-air via the so-called cloud.
The end result would be an iOS experience that's more like Android or Windows Phone out of the box—you pull it out, punch in your iTunes ID and you're doing magical things immediately, without having to sync to a computer running the latest version of iTunes on the latest version of OS X or Windows. And that'd be just part of iCloud.
Taken with the rumours about Time Capsule and iTunes music streaming, the big picture of iCloud is no less than the storage and connection of all your important data (documents, music, whatever) across all of your devices. Access to everything on your Mac through Time Capsule. Streaming your music through iTunes. Your contacts and calendars synced on every device. And more. It could be huge. [Daring Fireball]