Both of these planes are very similar and based on the same design. The MD-11 is just a more modern version. There are a couple of major differences that are visible to spotters. All MD-11s have large wingtip extensions called winglets. Also, the tailcone
at the rear of the MD-11 fuselage is wedge shaped while it is rounded on the DC-10.
An MD-11 is 21ft longer than a DC-10.
The DC-10 was originally available as a freighter but only a small number were built. Many more were converted from passenger to freighter configuration later. The MD-11 was also available as a freighter from the factory. In addition, all the original DC-10s that remain in service have undergone a cockpit upgrade to digital instrumentation with the new designation MD-10.
The Air Force version of these jets is called the KC-10 Extender and is based on the DC-10 freighter with a refueling boom added under the tail.