Different kinds of tags. The "tag" that "Tag cloud" refers to are short usually single-word textual descriptions of content. For instance, you've tagged this question with "google" "marketing" "tag-cloud" and "google-tag-manager".
The "tag"s that are managed by Google Tag Manager are HTML tags for services like Google Analytics or other services that provide you with HTML that you need to include in your site to work.
The two have nothing to do with one another, other than the word "tag" which has many meanings.