jBPM is about Business Process Modeling. Processes are defined using BPMN2 language.
Drools is about Business Rules. It is basically a Rule Engine where you can define those rules using a propietary language: DRL.
Version 5 of jBPM is based on an former module of Drools called drools-flow. That is why both projects share a lot of their base code.
You can use Business processes wihtout have to write any rule or you can use rules wihtout any process.
But a nice feature of jBPM is that you can mix both approaches and have some kind of interaction between both engines.
Best Regards,