Getting Started With the Java Rule Engine API (JSR 94): "Getting Started With the Java Rule Engine API (JSR 94): Toward Rule-Based Applications
By Qusay H. Mahmoud, July 26, 2005
For many mission-critical applications, the process of automating business policies, procedures, and business logic is simply too dynamic to manage effectively as application source code. Using business rules can help you develop more agile applications. The Business Rules Group defines a business rule as a statement that defines or constrains some aspect of the business; a business rule is intended to assert business structure or to control or influence the business's behavior. A rule engine evaluates and executes rules, which are expressed as if-then statements. The power of business rules lies in their ability both to separate knowledge from its implementation logic and to be changed without changing source code."
Site has links to Jrules, Drules, and Jess, amonst others.