A finite state machine (FSM) is a computational model used to design algorithms and systems. It consists of a finite number of states, transitions between those states, and actions that occur based on inputs. FSMs can be represented visually with state diagrams, making it easier to understand how the system behaves in response to different inputs. FSMs are widely used in various fields, including computer science, engineering, and linguistics. They can model simple systems like vending machines or more complex systems like computer programs and network protocols. Their simplicity and effectiveness make them a fundamental concept in automata theory.