Each iOS app can use iCloud storage to store documents associated to that app. So Pages for iOS will store its documents in the iCloud, and every other iOS device you have connected to that same iCloud account will more or less instantly see all those files. You can also access all the files stored by these iOS apps on icloud.com to download them manually to your computer.
As far as I know iCloud will also be supported by Apple's iWork desktop applications (Pages, Numbers, Keynote) in the future, but at this point they're still working on that. So for now you'll have to manually download files you've created on your iOS device and that are stored in the cloud to your desktop using icloud.com, and after editing them you need to manually upload them back to into the cloud using the same website.
And yes, that sucks. But hopefully soon, all that will be fully integrated. Also, Third party Mac applications that have an accompanying iOS app will probably get updates too so that it syncs their documents to both your iOS devices and desktop.
Bottom line: it'll be great, magical, fantastic... but just give a bit of time