What is the difference between a "salt box" style house and a "cape cod"?

Cape Cod architecture is very symmetrical in appearance with even sides, windows placed evenly on either side of the centered front door. Once inside you'll usually find a central staircase leading to the second floor.
https://en.wikipedia.org/wiki/Cape_Cod_%28house%29#Colonial_Capes_.2817th_century.E2.80.9318th_century.29

Saltbox architecture is very distinctive with the rear side of the roof sloping down from the second floor to the first floor of the house. That feature is the hallmark of a Saltbox.
https://en.wikipedia.org/wiki/Saltbox

http://architecture.about.com/od/housestyles/a/Cape-Cod-House-Style.htm