State diagram

A state diagram is an fundamental concept in computer science. States are nodes that have arrows that leave them for other notes. There is typically a unique starting state. Transitions are caused in general by some event. Often events are represented by particular messages. In the example, the messages a,b and c causes transitions between states in our set of states: {Start, State_1, State_2, Stop}. Our message alphabet is the set: {a,b,c}.

In Standard Unified Modeling Language, the start state is a solid circle. The end state is a solid circle with a circle around it. States are rounded rectangles.

              /--------\         /--------\
    ● -----> | State_1 | -----> | State_2 | -----> ◉
        a     \--------/   b     \--------/    c 

Also see

Finite state machine
Automaton