A tag cloud is a alphabetical list of tags, with their font sizes relative to how frequently they are used. Tags shown in larger fonts are used more frequently than tags shown in smaller fonts.
 LibraryThing Tag Cloud
http://www.librarything.com/tagcloud.php shows the Tag Cloud for all of LibraryThing - not every tag used by every person, but the top 1000 (?) tags used site-wide, sized by frequency. There is a link at the top to your tag cloud.
 Individual Tag Cloud
If the URL has the form /tagcloud.php?view=YOURUSERNAME, the tag cloud will show your personal tag cloud, which includes all of the tags you've applied to your books.
You can look at other people's tag clouds by modifying the URL, or by clicking on the "tag cloud" link from their profile.
Individual tag cloud pages have a link to the LibraryThing Tag Cloud. They also have the top navigation bar with links back to your profile, connection news, recommendations, reviews, author cloud, tag mirror, author gallery, and stats.