A state machine is a computational model used to design systems that can be in one of a finite number of states at any given time. It transitions between these states based on specific inputs or events, allowing for predictable behavior. State machines are commonly used in software development, control systems, and digital circuits. There are two main types of state machines: finite state machines (FSMs) and pushdown automata. FSMs have a limited number of states and transitions, while pushdown automata can use a stack to manage additional information. Both types help simplify complex processes by breaking them down into manageable states and transitions.