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.
If you're not using iCloud, iTunes also handles syncing those things to the iPhone and iPad, as well as ripping CDs, converting audio files, and more.
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 to sync your Mac's music to all your devices; and Apple Music, the company'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!