It looks like Apple hide a Dropbox-like folder in iCloud:
If you want to try out the trick yourself, make sure “Documents And Data” is turned on within iCloud settings, then navigate to Library inside your Home folder, and then find Mobile Documents.
Then, throw some files into the folder. These files should sync to any Macs where you’re signed in using the same iCloud credentials.
Right click to create an “alias” of the Mobile Documents folder on your desktop, and you’re good to go.
(via Clayton Morris)