Basically, Prolog is theorem prover à la Robinson, restricted to the Horn clause subset of first-order logic. Its programs are made up of implication rules:
conclusion0 if condition1 and condition2 and ... conditionn.
to prove conclusion0,
prove condition1 then prove condition2 then ... prove conditionn.
To demonstrate a goal, the engine follows a linear proof strategy, with backtracking.
"Most Prolog systems in the world owe their parentage to the original Marseille interpreter" (W.F. Clocksin & C.S. Mellish).