stevetuna . . .
Imagine firing a typical Smith, Colt or Ruger revolver (SA/DA), without ever cocking the hammer. Now imagine this in an autoloader. That's DAO (double action only).
Principal pro (as stated above) is single, consistent trigger pull/feel. Biggest con (in my opinion) is the longer, heavier trigger pull in comparison, for example, to a crisp, light, 1911A1's single action.