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A FORCE PLAY is a play in which a runner legally loses his right to occupy a base by reason of the batter becoming a runner. 

A DOUBLE PLAY is a play by the defense in which two offensive players are put out as a result of continuous action, providing there is no error between putouts.

(a) A force double play is one in which both putouts are force plays.

(b) A reverse force double play is one in which the first out is a force play and the second out is made on a runner for whom the force is removed by reason of the first out.

Examples of reverse force plays: runner on first, one out; batter grounds to first baseman, who steps on first base (one out) and throws to second baseman or shortstop for the second out (a tag play).
Another example: bases loaded, none out; batter grounds to third baseman, who steps on third base (one out); then throws to catcher for the second out (tag play).

Force Double Play
Runner from second base is tagged by the third baseman (5U) who scooped up the grounder, then the ball is thrown to the second baseman (5-4) for the force out at second. 

Reverse Force Double Play
First baseman caught the fly ball (F3) then threw the ball to the shortstop (3-6) who stepped on second base before the runner could get back to retouch.