In System Preferences > iCloud on the Mac and Settings > iCloud on the iPhone and iPad you can select which applications will use iCloud from among the following...
- Contacts (Addressbook)
- Calendars (iCal)
- Reminders (a separate app from calendar on the iPhone and iPad)
- Bookmarks (Safari of course)
- Notes (Stickies)
- Photo Stream
- Documents and Data
- Find my iPhone/iPad/Mac
When using Pages, Numbers, or Keynote the user must make the decision on whether to store the document(s) on iCloud or on the computer. SInce the iPhone and iPad have no data file system per se
data storage on those devices is in the form of open documents in the app. Files are saved on iCloud. There are several other third party apps such as ByWord that can make use of iCloud data storage, in fact some of those third party apps are almost pointless without iCloud. But in general you have pretty much full control over what goes into the cloud. You probably do not want to physically store much data on the iPhone/iPad itself because of the limited storage space, not to mention the possibility of complete data loss in the unlikely event of a crash.
ITunes has an option to store your tunes on iCloud as well. If they are available in iTunes the tunes files are physically located in the iTunes Store database so they occupy virtually zero space on your iPhone, iPad, or your iCloud account.
A final personal note, be sure to purchase a cover/case that protects the corners. The glass on the face and back is incredibly tough (I accidentally ran over an iPhone and it survived without ill effect) but dropped another that landed on a corner and the glass face broke from that corner. And don't forget the Applecare. It won't cover a dropped iPhone or one that a teenager had in his pocket when he jumped into the swimming pool, but still well worth the cost.