You can also search for iTunes U content using the search field at the top right of the iTunes window. On the results page, click iTunes U under FILTER BY MEDIA TYPE to just show iTunes U content.
Course content can vary from a lecture series like University of Minnesota Bookstore Author Events to full blown classes with video lectures, downloadable handouts and even required textbooks like 5th Grade Math.
Since iTunes U courses are really just enhanced podcasts, you can watch / listen to them on your Mac, iOS device or even a regular iPod for audio only courses.
For iOS devices, there is an iTunes U app that organizes your classwork into notebooks for your lecture, handout and textbook content. The iTunes U app can be very helpful for more involved courses.
While all courses on iTunes U are free and the course materials come from schools ranging from Harvard and Stanford to the Utah Electronic High School, there isn't any way to get actual credits for any of the courses on iTunes U.