An Event is a BPMN process modeling element representing something that “happens” during the course of a process. Events affect the flow of the process.  Some events such as most start events and some intermediate events have a “trigger” which defines the cause for the event, while end events often define the “result” or consequence of the ending of a particular sequenced flow.

Start Events can only “catch” a trigger. Similarly, End Events can only create “throw” a result. Intermediate events are less constrained and may catch triggers or throw results.

The symbol used for an event is always an unfilled circle.  However, there are several other standard attributes of the event symbols that are of importance.

• Internal Markers - many times the circle will have an icon or “marker” within it providing more information about the type of event. 
  
• Filled versus Unfilled Markers - the internal marker may be filled or unfilled.  Unfilled internal markers are used to identify that the event is a “catching” event.  This means that the event is waiting to receive notification that something has happened.  Filled internal markers are used to identify that the event is a “throwing” event.  This means that the event is being generated at that point in the process.
  
• Single, Double, or Filled Outer Circle – the outer border of the event may be a single circle, a double circle, or a filled ring.  A single thick circle means that the event is a starting event, used to signify a start of a process.  A double circle means that the event is an intermediate event, used for any event which occurs after the start of a process but before its completion.  A filled ring means that the event is an end event, used to signify the end of a process.
  
• Dashed versus Solid Circle – both start and intermediate events can be show with the outer circles being solid lines or dashed.  Solid lines are used to show interrupting events.  These are events which are triggered the moment the event occurs regardless of whether the activity has completed.  Dashed lines are used to show non-interrupting events. These are events which will wait for the completion of the activity before flow of the process continues.