iTunes is so many things. It's a Mac — and Windows! — app that organizes and plays music and video and even sells them, as well as books, podcasts, and apps, through the iTunes Store. iTunes on the desktop also handles syncing all those things to the iPhone and iPad — if you don't want to us iCloud — as well as ripping CDs, converting audio files, and more.
On iOS, iTunes is broken up into the iTunes Store and the matching Music and (http://www.imore.com/video-iphone-ipad) apps, App Store, iBooks, Podcasts, iTunes U, and even Apple's Movie Trailers app.
You can find iTunes Preview pages on the web, content providers use the iTunes CMS to manage their media, and developers have iTunes Connect to manage their apps and games.
iCloud services for iTunes include iTunes in the Cloud, which lets you re-download music, TV shows, movies, and apps from any device logged into your iCloud. iTunes Match which lets you pay $25 a month for a music locker, and iTunes Radio which is Apple's streaming music service.
And that's still only scratching the surface of all the features iTunes provides and all the ways it connects Apple's OS X and iOS devices together!