Some information about forum related content classes you can find in the Website Interface user guide: http://ez.no/doc/content/download/22884/95262/version/4/file/ezwebin_userguide-1.3.pdf
What exactly do you want to know about tag clouds in eZ Publish? Technically tag cloud is provided by template operator in eZ Publish templates which returns cloud of words (in different size) to the end user. Those words are simple links pointing to the content/keyword view where you can see all content related to selected keyword.
