If you don't know what Tag Cloud is, please read
Wikipedia article. Here is the example of the Tag Cloud.
To add the Tag Cloud to your site, please go through the following 2 steps.
1) Add Tag Cloud block to your page.
Go to any page that you wish to add Tag Cloud,
enable the page editor. Click "Add" icon and select "Tag Cloud"
2) Add tags to your articles.
When you
add/edit articles assign tags to the articles. Tag Cloud will be updated automatically.