I'm looking for a good rule engine.
I've seen it exists two good projects : JBPM and Drools.
I don't really understand differences between the two and I don't know what is the ideal tool to use.
Please could you provide me more information and your experience with one or other and what do you think is the best to use to manage business rules.
jBPM is not a rule engine, it's a workflow engine. Drools is a rule engine. So Drools is what you're looking for.
Drools and jBPM are companion projects: they integrate really nicely if you need workflows with rules.