Expert systems are intelligent systems used to get specialized information in a very specific and limited domain of human expertise. An expert system captures the knowledge of skilled employees and experts in the form of a set of rules in a software system that can later be used others.
Expert system can use a wide variety of methods to simulate the performance of the expert. Most common of these methods are 1) the creation of a knowledge base which uses some knowledge representation formalism to capture the Subject Matter Expert’s (SME) knowledge and 2) a process of gathering that knowledge from the SME and codifying it according to the formalism, which is called knowledge engineering. Expert systems may or may not have learning components (or artificial intelligence) but a third common element is that once the system is developed it is proven being placed in the same real world problem solving situation as the human SME, typically as an aid to human workers or a supplement to some information system.
In actual decision making in the context of difficult and new problems, expert system may be of little use but for well-defined problems with well-defined solutions, expert systems can give quick answers based on the already existing knowledge base.