Tag Clouds are a 'new' way to display information. Specifically, it is a visual way to depict the occurrences or popularity of a specific word. Apparently this started with Flickr, although it is now common, especially among Web 2.0 companies.
I really dislike tag clouds. OK, they aren't really offensive, but I'm a numbers guy, and I'd much rather see the information displayed in a table or graph than as a tag cloud. I don't find it really accurate or useful.
Like any good fad, they are now being applied to everything you can think of. For example, a tag cloud of presidential speeches.