A tag cloud (also known as a word cloud, wordle, or weighted list in visual design) is a visual representation of text data, which is often used to depict keyword metadata on websites or to visualize free-form text. Tags are usually single words, and the importance of each tag is shown with font size or color. When used as website navigation aids, the terms are hyperlinked to items associated with the tag.
Tag clouds are typically represented using inline HTML elements. The tags can appear in alphabetical order, in a random order, they can be sorted by weight, and so on. Sometimes, further visual properties are manipulated in addition to font sizes, such as font color, intensity, or weight. The most popular is a rectangular tag arrangement with alphabetical sorting in a sequential line-by-line layout. The decision for an optimal layout should be driven by the expected user goals. Some prefer to cluster the tags semantically so that similar tags will appear near each other or use embedding techniques such as tSNE to position words. Edges can be added to emphasize the co-occurrences of tags and visualize interactions. Heuristics can be used to reduce the size of the tag cloud whether or not the purpose is to cluster the tags.
Tag cloud visual taxonomy is determined by a number of attributes: tag ordering rule (e.g. alphabetically, by importance, by context, randomly, ordered for visual quality), the shape of the entire cloud (e.g. rectangular, circle, given map borders), the shape of tag bounds (rectangle, or character body), tag rotation (none, free, limited), vertical tag alignment (sticking to typographical baselines, free).
A tag cloud on the web must address problems of modeling and controlling aesthetics, and constructing a two-dimensional layout of tags, and all these must be done in a short time on a volatile browser platform. Tags clouds to be used on the web must be in HTML, not graphics, to make them robot-readable, they must be constructed on the client side using the fonts available in the browser, and they must fit in a rectangular box.