A rule-based system is a system that uses a set of predefined rules to make decisions or solve problems. These rules are typically based on if-then statements, where the system will take a specific action if certain conditions are met. Rule-based systems are often used in applications such as expert systems, decision support systems, and automated reasoning systems. The main advantage of a rule-based system is that it does not require a training set. The main disadvantage of rule-based systems is that they can become very complex and difficult to maintain as the number of rules increases.