In a sense, this is like asking the difference between "fruit" and "kiwi friut". Just as a kiwi fruit is one type of fruit, so a revenge tragedy is one type of tragedy. In other words, although not all tragedies are revenge tragedies, all revenge tragedies are types of tragedy.
The term "revenge tragedy" applies to a specific group of plays written during the Renaissance, influenced by the Latin tragedies of Seneca and sharing several common characteristics. First, the protagonist of the play is normally someone who was wronged in the past, and is seeking revenge. Next, there is normally an antagonist in a position of authority who is evil or corrupt. English revenge tragedies are usually set in distant times and places. The plots tend to be convoluted and melodramatic, and the the plays usually end up with multiple people murdered or committing suicide. Some examples of this genre include Hamlet, The Revenger's Tragedy, Antonio's Revenge, and The Spanish Tragedy.