iTunes in the Cloud is part of Apple's iCloud service and makes it easy for you to download any purchases you make across all of the devices you have authorized on your account. For example, if you download a book on your iPhone, you can automatically have it show up on your iPad. The same holds true for apps and music. The other incredibly convenient portion of iTunes in the Cloud is the ability to stream movies, music, and tv shows to your iPhone, iPad, iPod touch, or Apple TV without actually having to download anything.
If you've ever seen a cloud download arrow next to an app, game, song, album, tv show, movie, or book that you previously purchased, you're already familiar with iTunes in the Cloud. At its core it's simply a way to tie all your purchases together for convenience. You can take iTunes in the Cloud one step further by subscribing to iTunes Match, which gives you the ability to match your entire music collection to an iTunes track that will then be available for streaming. This is an ideal option for anyone with larger music libraries and not enough space on their iPhone, iPad, or iPod touch to store it all.
After launching iOS 8, Apple also announced a new feature called Family Sharing that allows families to add up to 6 people to a sharing group. Those people then have access to each other's iTunes in the Cloud purchases without having to have the same iTunes ID signed into each device.
While iTunes in the Cloud offers a lot of convenience, certain features are only available in certain countries, such as iTunes Match. So if you're missing settings or don't think iTunes in the Cloud is working properly for you, you can check Apple's official availability page to see what features are supported currently in your country.