In ABAP/4, the event immidiately begins when the program is running. In general, these events is normally used in the report:

• INITIALIZATION
  This is the first event when the report generated. We can modify default value of selection screen.
• AT SELECTION-SCREEN
  If we want to verify input data before executing program, we will use this event. This event is triggered when user press Enter.
• START-OF-SELECTION
  This event is activated at the main process when the program is executed.
• END-OF-SELECTION
  This event is activated follow on START-OF-SELECTION event.
• TOP-OF-PAGE
  We use this event for page heading report.
• END-OF-PAGE
  We use this event for page footer report.
• AT LINE-SELECTION
  When user double-click the line, this event will be activated.
• AT USER-COMMAND
  This event will be activated when user exexutes a function within menu