An expert system is a computer-based system that uses existing human expertise in a specific domain to solve problems. It stores knowledge about the domain and uses rules and reasoning to answer questions and produce solutions. The expert system gathers knowledge from experts to build its knowledge base and then asks a series of questions to solve problems, using the stored knowledge and reasoning capabilities. Unlike problem-solving programs, expert systems are not designed for a single problem - they acquire expertise to handle a broad range of issues in the domain.