iTunes™ is a software application developed by Apple. It is available for Mac OS and Windows and can be downloaded freely from apple.com.
iTunes' original purpose was to allows you to manage your digital music library (mp3, aac, etc.), listen to music on your computer and synchronize your music with your iPod.
iTunes evolved into a media manager not just handling music/"tunes" but also Movies, TV Shows, Podcasts and more.
iTunes is required to access the iTunes store which lets you purchase music, videos and othe rmedia but also applications for the iPhone, as well as ringtones.
More info: http://www.apple.com/itunes/download/