Expert systems are intelligent computer programs that use knowledge and inference procedures to solve problems that require significant human expertise. An expert system consists of a knowledge base containing expertise and an inference engine that draws conclusions. Expert systems can assist humans, replace humans when experts are unavailable, and make consistent decisions. Examples of applications include accounting, geology, and medical diagnosis systems. Expert systems provide benefits like consistency and always being available but also have limitations like a narrow problem scope and inability to refine their own knowledge.