It's either a shell replacement or a desktop overlay.  They both do basically the same thing.  A shell replacement would overwrite or replace system files to make the desktop customisable, whereas a desktop overlay just covers up the desktop with something else and is less permanent.  I think Rainmeter is the latter of the two.  

Years ago when I used HoverDesk (an older desktop overlay that seems to have vanished from the web), my desktop looked like this:



Think that's from 2004, so about a decade ago now.  Just before everyone stopped using Internet Explorer and hardly anyone had widescreen monitors, heh.