A topic cloud, also known as a tag cloud, is a visual representation of the written data on a website. The topic cloud takes the form of a list of short tags which briefly describe the issues discussed on a website. The more often a tag is used, the bigger and bolder it gets. A casual glance at a topic cloud shows users which topics are discussed more more frequently. Clicking on an individual topic usually results in a change of navigation, such as opening up a list of articles which discuss that topic, or the creation of another topic cloud.

This type of visual representation is often called a weighted list in graphic design, because while providing a list of tags, it also provides meta data. Meta data is data about data. A static list of tags will help users determine roughly what a website is all about. A topic cloud, on the other hand, provides data about the tags. Using a topic cloud can sometimes reveal trends about a website which are not readily apparent from a casual glance. It is also a fun tool for website owners to use, especially if they are curious about long term trends on their websites.

Bloggers use topic clouds extensively, creating a list of keyphrases about things which they cover frequently so that visitors can quickly assess whether or not they want to stay on the blogger's site. News aggregation sites also utilize the concept, so that users can see which news is “hot” at the moment. The keywords in a topic cloud will also show up on Internet searches, if the topic cloud has been well coded. Internet users who perform a search for a particular topic can access a website through its topic cloud.

The most basic type of topic cloud is a static illustration of topics. However, most designers make their topic clouds clickable, allowing users to interact with the topic cloud. A news site, for example, might bring up a list of articles related to the topic the user clicks on. Other designers create a series of nested topic clouds. A user might start, for example, with a site-wide topic cloud. If the user sees an issue of interest and clicks on it, the issue of interest becomes the center of a new topic cloud which contains related topics. From there, the user can explore sections of the site which deal with these topics.