A tag cloud or word cloud (or weighted list in visual design) is a visual depiction of user-generated tags, or simply the word content of a site, used typically to describe the content of web sites. Tags are usually single words and are typically listed alphabetically, and the importance of a tag is shown with font size or color. Thus both finding a tag by alphabet and by popularity is possible. The tags are usually hyperlinks that lead to a collection of items that are associated with a tag.
A tag cloud is a set of related tags with corresponding weights. Typical tag clouds have between 30 and 150 tags. The weights are represented using font sizes or other visual clues. Meanwhile, histograms or pie charts are most commonly used to represent approximately a dozen different weights. Hence, tag clouds can represent many more weights, though less accurately so. Also, frequently, tag clouds are interactive: tags are hyperlinks typically allowing the user to drill down on the data.
Prior to weighted list representation of tag clouds, paper maps had used the concept of weighted font size and font weights to represent relative size or importance of towns and cities.
There are three main types of tag cloud applications in social software, distinguished by their meaning rather than appearance. In the first type, there is a tag cloud for each item whereas in the second type, we have global tag clouds where the frequencies are aggregated over all items and users.
In the first type, size represents the number of times that tag has been applied to a single item. This is useful as a means of displaying metadata about an item that has been democratically 'voted' on and where precise results are not desired. A good example of this is Last.fm, which uses this method as a means of displaying the genre with which an artist or track has been tagged.
In the second, more commonly used type, size represents the number of items to which a tag has been applied, as a presentation of each tag's popularity. Examples of this type of tag cloud are used on the image-hosting service Flickr and the blog aggregator Technorati.
In the third type, tags are used as a categorization method for content items. Tags are represented in a cloud where larger tags represent the quantity of content items in that category.
More generally, the same visual technique can be used to display non-tag data, as in a word cloud or a data cloud.
Tag clouds are typically represented using inline HTML elements. APML (Attention Profiling Markup Language) is often used to store and import/export Tag Clouds. The tags can appear in alphabetical order, in a random order, they can be sorted by weight, and so on. Some prefer to cluster the tags semantically so that similar tags will appear near each other. Heuristics can be used to reduce the size of the tag cloud whether or not the purpose is to cluster the tags.
A data cloud is a data display which uses font size and/or color to indicate numerical values It is similar to a tag cloud but instead of word count, displays data such as population or stock market prices.
A text cloud or word cloud is a visualization of word frequency in a given text as a weighted list. The technique has recently been popularly used to visualize the topical content of political speeches.