Knowledge Based Systems (KBS) and Artificial Intelligence (AI) Tools
The term Knowledge Based System (KBS) is more general than the notion of an Expert System, but both are classed as AI (Artificial Intelligence) tools. Although almost all commercially available expert system shells allow external programs and databases be used, the expert systems are centred mainly around heuristic knowledge. However, the real-world domains are more complex and the problem solving there requires, as a rule, an intensive usage of different kinds of knowledge. Thus, KBSs are not centred around one kind of knowledge. To the contrary, the declarative, procedural, and heuristic knowledge in such systems have the same rights and significance in the process of decision making. In summary every ES is a KBS, however, the same does not apply vice-versa.
If you want to know more you might want to visit the downloads page and download EAPSTRA Deliverable 1 that gives details of Knowledge Based Systems.