An expert system is made up of three parts:
A user interface - This is the system that allows a non-expert user to query (question) the expert system, and to receive advice. The user-interface is designed to be a simple to use as possible.
A knowledge base - This is a collection of facts and rules. The knowledge base is created from information provided by human experts
An inference engine - This acts rather like a search engine, examining the knowledge base for information that matches the user's query