GameMaker is completely event driven, and all actions happen as the result of certain events that are triggered in a specific order or by a specific occurrence within the game. There are a number of different events, each one with a specific task or timing and some of them are even split into separate "sub" events. Normally these events are run by GameMaker when it detects an in-game trigger, or every game tick (as is the case with the step and draw events), but you can also apply an event to an instance in the current room from within a piece of code.
NOTE For more information on events please see the section on Object Events.
NOTE These variables are read-only.