Drools, formally called JBoss Rule(http://www.jboss.org/drools/). Its a best Rule Engine in open source community. So what  it is about.

Very simple

1. You have rules like if then (.drl file)
2. You have Java Object
3. Submit java object to rule engine

For example

Employee emp = new Employee(1, ‘Waqas Noor’)
ruleengine.submit(emp) ;

ruleengine is like

rule
     when 
            emp : Employee (name=’Waqas Noor’) 
    Then
            System.out.println(“Its so simple”)
end

The .drl translated into like

—– Code —-
if(emp.getName().equals(“Waqas Noor”))
        System.out.println(“Its so simple”); 

 its just a start; there is much to do; its really a fun to work with drools, visit for more detail (http://www.jboss.org/drools/)