You've probably noticed it in the navigation bar at the top of the iTunes Store--sandwiched between Podcasts and the much-maligned Ping--but have you ever clicked on the iTunes U link? Millions of people have, and iTunes U has seen more than 600 million downloads since its inception in 2006.

An integral part of the iTunes Store since May 2007, iTunes U is an extraordinary repository of educational content from more than 1,000 universities around the world. Here's how it works.

When you go to the iTunes U section of the iTunes Store, you'll see that it looks a lot like the rest of the store. There are rotating graphics at the top for featured content, promotional bricks in the middle of the page for different themes, and "top charts" showing the most popular downloads. But what's less obvious at first glance is the fact that all this content is free.

Much as with podcasts, you can either view or listen to iTunes U content directly, download it, or subscribe to specific courses. For a course that has completed, you can download all the content, or view individual items whenever you want; for courses that are active, you can subscribe so you get the latest material as soon as it's made available.

As I write this article, there are featured sections for courses about Shakespeare, ethics, playwriting, American history, algorithms, foreign languages, and animal physiology. Pretty much anything you'd find in a college course catalog shows up on iTunes U.

You can browse the featured topics, browse by category, search the featured providers (in the right-hand sidebar, when you scroll down a bit), or perform a keyword search from the Search Store box in iTunes's upper-right corner. Search results will show all types of content that match your search--movies, music, books, and so on--but just click the iTunes U link in the Filter By Media Type box that displays in the upper-left corner of iTunes after you've performed your search to see only that type of content.

Each person will find different types of content that interests them on iTunes U, but it's safe to say that anyone who wants to learn will find something to suit them. My iTunes U library contains lectures about Shakespeare's plays; a series of Smithsonian Folkways recordings about American folk music; Michael Sandel's popular and interesting lecture series, Justice; a series of lectures, in French, about Marcel Proust's novel À la recherche du temps perdu; and a number of courses about philosophy.

iTunes U content comes in several types. There are audio and video lectures, but there may also be PDFs or ePub files. (For example, Oxford University provides all of William Shakespeare's plays, from the First Folio edition, in ePub format in "old spelling" as a course.)

You can view videos or listen to audio lectures directly by clicking on a link on a course page, or download the content by clicking on Get All. You can also subscribe by clicking Subscribe Free, or download a single item by clicking on the Free button to the right of the item.

You can also view or listen to content from an iOS device by going to the iTunes app and tapping on the iTunes U icon on the iPad, or by tapping on More -> iTunes U at the bottom on the iPhone or iPod touch. Tip: If you become a big fan of iTunes U, you can even add the iTunes U link to the bottom of your iTunes app on the iPhone. Just tap the More link, then the Edit button that appears on the top-right corner of the More screen. You'll be taken to a Configure screen where you can drag the iTunes U icon on top of one of the other icons already at the bottom to replace it with iTunes U. (You can actually use this method to customize the app's buttons in general, replacing or moving the four buttons that appear before More to suit your needs.)

Managing your iTunes U content is very similar to the way you work with podcasts. This content shows up in its own library in the iTunes sidebar, and you can sync it to your iPod or iOS device, controlling syncing options from the iTunes U tab when that device is connected.

There are several options for managing how iTunes gets new content. To view these options, go to your iTunes U library, then click on Settings at the bottom of the iTunes Window. (If you don't see an iTunes U entry in your iTunes sidebar, go to iTunes's General preferences and click the box next to iTunes U in the first section of prefs to put a check mark next to it, then click OK at the bottom.)

By default, iTunes checks for new iTunes U content every day, downloads only the most recent episode, and keeps all episodes. You can change these settings for all iTunes U courses, with the Settings For iTunes U Defaults option--choosing to download, say, all new episodes and keep only those that are unplayed. Using the Settings For pop-up menu, you can choose to apply specific settings to individual courses; say you want to keep all episodes of most courses, but not of a specific lecture series that you have only a passing interest in following; you can make the change by choosing the one you want to change from this menu.

Obviously, iTunes U won't replace a college education, and you can't (yet) get a degree from it, but many universities use iTunes U to distribute content for actual courses that do offer degrees. If you're not a student, the ability to essentially audit thousands of courses for free is invaluable. And for anyone interested in learning, iTunes U provides content about just about every subject imaginable. Take advantage of iTunes U to broaden your horizons.