Drools is an Object-Oriented Rule Engine for Java. Drools is an augmented implementation of Forgy's Rete algorithm tailored for the Java language. Adapting Rete to an object-oriented interface allows for more natural expression of business rules with regards to business objects.
More importantly, Drools provides for declarative logic programming and is flexible enough to match the semantics of your problem domain. The various parts that compose a rule may be extended to lend domain-specific sense to the rule.