iTunes is a free application from Apple that can be used with Mac or PC. It plays all your digital music and video on your computer. It can sync content to your iPod, or iPhone. It is not a web site, but can access the web. It is a software application (among others known as podcatchers) that can automatically identify and download new files associated with the podcasts and/or play and manage directly downloaded video, audio or pdf files.
Within iTunes is the iTunes Store, accessed via the web, where users can listen to/watch samples of music, videos, movies and purchase them. There are also many free podcasts users can subscribe to. Within the iTunes Store, is iTunes University where many of the worlds major educational institutions have there content. While many of the files in the iTunes Store are for purchase, the files in iTunes U are free.
For more information: visit About Apple iTunes | Watch a Video Overview | Download iTunes