Prolog is a logic programming language invented in France.
Prolog programs consist of rules and facts. Rules are executed in a way called backward-chaining. Prolog programs generally deduce facts using deductive logic, as opposed to forward chaining - which is what CLIPSLanguage uses.
Prolog programs are very handy for writing lightning quick in-memory databases. The data retrieval speed in a good Prolog implementation like SWI-Prolog is mind-shattering.