A lot of Drools is based on work first done in CLIPS, the LISPy rules engine of the 80s and early 90s. Strangely, I already knew about CLIPS. They are all based around the Rete algorithm, a clever way to trade computation for memory with if-then-else rules systems.
I learned about CLIPS in the late 80s from Brian Donnell (now Dantes), a fine fellow. We were classmates together at in Baker College at Rice University, and in fact he was the very first person to ever explain Object Oriented Programming to me. (As he wrote COOL, the OOP system in CLIPS, this was a topic he enjoyed discussing.) He tried describing it in context of C with Classes and C++; at that time the only language I knew was C. I can thank him and Matt Cohen (who taught me C) for starting my life in programming.