Hello there, fpefpe.
The following links provide some great information on how iCloud works:
iCloud - All y our content on all your devices.
https://www.apple.com/icloud/features/
With iCloud, anything you purchase in the iTunes Store, the App Store, and iBooks is automatically available on all your devices the moment you tap Buy. You can also access past purchases from any of your devices.
If you want to have your non-iTunes purchased content in the cloud, then you may want to look into iTunes Match. These articles provide great information on the service as well:
iTunes Match
https://www.apple.com/itunes/itunes-match/
and
iTunes Match on iPhone, iPad, or iPod touch
http://support.apple.com/kb/HT5637
Thanks for reaching out to Apple Support Communities.
Cheers,
Pedro.