How does Windows Malicious Software Removal Tool work?

This is one of this things that is always updated on “Patch Tuesday” but as far as I know I have never used it, nor has it removed anything. How does the Malicious Software Removal Tool work? Does it just run in the background without any required input from me, or do I need to do something that I am not currently doing?

Story added 27. January 2014, content source with full text you can find at link above.