A game, when running or being played, is a dynamic system. It is something that changes, often quickly from the player point of view. These constant change in the game is actually the result of small very frequent changes, perceived as a greater, more complex turn of events. The key to determine how the game evolves is to understand the basics changes and how establish them from their components called "events" and "actions"

To better understand events and actions, we must first consider that a computer game is actually a program which is running continuously. Sometimes this program is known as "game engine". This program is frequently monitoring the keyboard, mouse, and other controllers; it can read the time, knows the position and direction of any object from the game, etc. Therefore, the game engine is the first one to know that something has happened: either a key has been pressed, or a delay has been elapsed, or two objects have touched in the screen, etc. GameMaker calls events to these things that occur and can affect the game. Many events come up as a result of the player interacting with the computer. Some other events occur as a result of the evolution of the game.

The game engine controls the game and may change the state of anything in it, at any desired moment. It can reposition or move an object, add some damage to it, create a new object, etc. These changes happens as intended by the programming done by the developer, which the engine game is only the executor of the programm. GameMaker calls "actions" to a change which is specified by the developer. The key here is that every action happens as a result of an event, even actions which are set to happen randomly are linked to events with a random component on them. The goal of the developer is to set which actions happens as a consequence of which events. We'll analyze events in depth first and then we'll see how actions are linked to them

There is a long list of pre-established events by Game Maker, but the game developer is free to take any action in response to any chosen event (please make reference to the Online resource).