Here is a page all about tags. They are the green boxes, and help narrow down and focus your weekly Feed (that you get by email) and G2G posts.
https://www.wikitree.com/wiki/Help:Tags#What_are_Tags.3F
Categories look like this:
[[Category:Devon, Haywood Name Study]]
and group together profiles all in that same category (who have the category on their profiles), such as all the Baptist preachers on WikiTree, all the racecar drivers on WikiTree, all the pastry chefs on WikiTree, and so on. They have nothing to do with your weekly Genealogist Feed nor G2G.