Go to Settings
Go to General
Scroll down to Restrictions
Enter and then Enable Restrictions
Set a password (will be required to re-enter restrictions menu and to disable restrictions)
And you're good! (Keep reading for what you can do)
You can allow specific things, like if you can delete/install apps, or FaceTime. If you disable iTunes, or camera, or Safari, the apps will disappear!
You can change what type of content you can download on the device and ratings from which country
You can also turn off any In-App Purchase. And you can change how long it takes for it to ask you for a password when downloading something (Ex. Asks for password 15mins after previous purchase)
You can change around privacy-related things
And you can put a maximum volume limit, if you can change accounts around (like mail accounts) and if you can add/play with others on Gamecenter
This is useful if the device is being used by kids, so they don't make a lot of purchases, add strangers, mess around with device, etc.
Also, if you don't use iTunes, why not disable it so the app goes away and it doesn't take up app space (Same for Safari/Camera, but no Safari= no opening links, and no Camera=no lock screen shortcut)