An expert system is a computer program that uses knowledge from domain experts to assist humans or make decisions. Some key expert systems include PROSPECTOR for mineral exploration, PUFF for respiratory diagnosis, and MYCIN for blood disorders diagnosis. Expert systems have a knowledge base of facts and rules, an inference engine to apply rules to solve queries, and a user interface. They are useful when human experts are unavailable, inconsistent, or too expensive. However, expert systems also have limitations like a narrow domain of knowledge, inability to learn, and legal/ethical concerns about responsibility.