Group:
A group of multiple URLs.
Groups can be used for filtering and sorting the ranking views and the reports, and for sub users permissions.
URL:
A web address (URL) that you want to track for specific terms and search engine combinations.
Read about PRT search / track logic — how a URL should be entered.
Term:
A term is a keyword or keyphrase that is tracked for the specified URL.
Your terms balance is counted as: 1 term = each URL + Term + Search Engine combination.
Read more about it.
Tag:
A tag is a group of terms.
A term can have multiple tags.
A tag can have terms from different URLs.
Terms of the same URL can have different tags.
Grouping terms in tags can be used for filtering and sorting: you can display grouped terms from different URLs, or inside one URL, either in the ranking views or in the reports.