Is there an explanation somewhere for the distinction between Mentions and Tags?

After much reflection, I thought this might just be a subject/object distinction (whom I mention/tag vs. who tags/mentions me). However, the global stats page treats tagged/tags and mentioned/mentions as four separate categories.

Similarly, the User Tagging settings page--which needs a better name, since it covers far more than tagging per se--clearly distinguishes between "Tags from [other] users" and "Mentions from [other] users."

It is entirely possible that I'm just being dense. That being said, FT has a lot of dense users for whom this dichotomy will likely be equally inscrutable.

Also: how does one tag/mention someone whose FT username contains one or more spaces?