A tag cloud is a sidebar widget that displays all of the categories or tags on your site, formatted so that the most commonly used taxonomies are the most visually prominent (see screenshot, below). Tags or categories that have no posts associated with them will not be displayed.
In this example, the most popular tag is News & Announcements, which has 118 posts, and the least popular (2 posts each) are Tutorials, Winter Study, and On Research.
Adding the widget
Use the widget editing interface (Appearance -> Widgets) to add a Tag Cloud widget to your sidebar.
The text that will be displayed in the dark blue widget header bar.
Select either tags or categories to be displayed in your widget. By default, tags will be used.
Click the blue “Save” button when you’re done.