It is a diagrammatic tool to model concurrency and synchronization in distributed systems. It is very similar to State Transition Diagrams. It is used as a visual communication aid to model the system behavior. It is based on strong mathematical foundation.

It consists of three types of components: places (circles), transitions (rectangles) and arcs (arrows).  The Places represent possible states of the system. The transitions are events or actions which cause the change of state. Every arc simply connects a place with a transition or a transition with a place.

A change of state is denoted by a movement of token(s) (black dots) from place(s) to place(s); and is caused by the firing of a transition. The firing represents an occurrence of the event or an action taken. The firing is subject to the input conditions, denoted by token availability. A transition is firable or enabled when there are sufficient tokens in its input places. After firing, tokens will be transferred from the input places (old state) to the output places, denoting the new state.