An expert system is a computer system programmed to use knowledge in the form of rules or frames to solve problems by capturing the expertise of a human expert in limited domains of knowledge and experience. Expert systems can interact with humans and consider multiple hypotheses simultaneously. The knowledge incorporated in an expert system must be supplied by a human expert. Such systems can't really think like humans and can merely assist decision making by asking relevant questions and explaining the reasons for adopting certain actions. It includes an inference engine or the strategy used to search through the rule base.
Rule base is a list of if-then rules which are interconnected.
A semantic net represents knowledge and expertise that is composed of easily identified chunks or objects of hierarchically interrelated characteristics. Semantic nets are more efficient than rules if the data can be represented this way. Frames also organize knowledge into chunks, but chunks that are related by shared characteristics. For example, cars and tanks are separate frames. Inference engine is the software that embodies the strategy used to search through the rule base.