Role Playing Games or RPGs as they are also called used to be the description of pen and paper games played in many young boys' basements, killing imaginary monsters with imaginary swords and spells.
Today the term mostly refer to PC games or video games where a player is in control of one or more characters, who over time grows stronger, and find better equipment to defeat stronger enemies. It's rarely much to do with actual "role playing" any more.
The Diablo games are classified as "Action RPGs", and has an even thinner slice of character improvement and management, and further focus on killing monsters. More action and less management has proven to be very popular.