Clusters of keywords. Seemingly random bunches of floating words. A tag cloud is a visual depiction of user-generated tags. Here are some interesting related sites. The first, TagCrowd, lets you make your own “tag cloud” from whatever words you enter. If you enter words more than once, the higher frequency is reflected in the larger size of the font.
Posts on Cloudalicious are a little infrequent, but paint a vivid picture of the evolution of the tag cloud concept. Interesting reading indeed!