From Wikipedia:Tag_cloud
A tag cloud (more traditionally known as a weighted list in the field of visual design) is a visual depiction of content tags used on a website. Often, more frequently used tags are depicted in a larger font or otherwise emphasized, while the displayed order is generally alphabetical. Thus both finding a tag by alphabet and by popularity is possible. Selecting a single tag within a tag cloud will generally lead to a collection of items that are associated with that tag.
The first tag cloud appeared on Flickr, the photo sharing site. That implementation was based on Jim Flanagan's Search Referral Zeitgeist, a visualization of web site referrers. Tag clouds have also been popularised by Technorati, among others.