DH and DA are different terms for the same thing - Decision Height or Decision Altitude. They are used for Precision Approaches (ILS) to denote the altitude which, upon reaching during the final approach, if the runway environment is not visual a missed approach must be conducted.
A MDA or Minimum Descent Altitude is the minimum altitude you are allowed to descend to on a non-precision approach. On a NPA the plate will describe the point at which you can descend to the MDA and then it will describe the point at which you must complete a missed approach, incidentally known as the Missed Approach Point.
Essentially, the DH/DA is the Missed Approach Point because you must go around at that altitude and you cannot fly along at it searching, while a MDA does allow you to fly at the altitude until either seeing the runway through a break in cloud or until you reach the missed approach point.
At the end of the day though, they are similar in that it is the lowest altitude you can fly to on an approach - its just the type of approach that denotes whether its aa DH/DA or MDA.