CDML tags are similar to HTML tags, but serve a different purpose. CDML tags are placed in the format files that are to be served to the web.
Variable tags tell the Web Companion how to perform actions.
Replacement tags are replaced with some sort of text, depending on the tag. It is the most used tag. Other tags will be replaced with things like the current found count, current record number, current client information, current token, cookies, database information, etc.