iTunes is a proprietary digital media player application, used for playing and organizing digital music and video files. The application is also an interface to manage the contents on Apple's iPod and iPhone lines, as well as the iPad. Please specify when you are using iTunes on a Windows computer, otherwise Mac will be assumed.
iTunes can connect to the iTunes Store to purchase and download music, music videos, television shows, iPod games, audiobooks, podcasts, movies and movie rentals (not available in all countries), and ringtones (only available on iPhone and iPod Touch 4th Generation). It is also used to download apps from the App Store for iPhone, iPod Touch and iPad.
iTunes runs on both Mac and Windows computers. Both are explicitly on-topic here, but if you are asking a question about Windows, please make sure you mark it as such or else you may get a Mac-specific answer.