Finite state machine (FSM)
Beginner
A circuit that moves step by step through a fixed set of modes, like a washing machine going from fill to wash to spin.
Novice
Control logic that is always in one of a fixed list of states and moves between them according to its inputs. It is built from a register holding the current state plus logic that computes the next state and the outputs.
Expert
Coded as a state register (always_ff) plus next-state and output logic (always_comb). The bit pattern chosen for each state (binary, one-hot, Gray) trades flip-flop count against logic depth and switching, and synthesis tools may re-encode the machine.