"Supercoiled" (or "Covalently Closed-Circular") DNA is fully intact with both strands uncut, and with a twist built in, resulting in a compact form, so they move faster others."Linear" DNA has free ends, it moves at the speed similar to its molecular weight, compared to the markers."Nicked Open-Circular" DNA has one strand cut, it is not supercoiled, and it is not compact form, but it is circular. Therefore, it migrates slower than the above 2 forms. please see the picture.