Greetings, Game Maker - The Game Maker's Companion: Game Development - page 5

Instances and Objects
Objects are the programmable elements of your game and their behavior is directly determined
by the events and actions that you choose to give them. However, there is an important
distinction to be made between the object resources , which define the general behavior of objects,
and the individual object instances , which occupy your game world. Once you have defined the
behavior of an object resource, then you can place any number of instances of that object in your
game. All these instances will behave in the same way (because they follow the same events and
actions), but each has its own variables (position, speed, direction, and so forth) that are unique
to that instance. If you like, you can think of object resources as being like jelly molds, and
instances as the jellies you make with them. You only need one mold to make any number of
jellies that have the same basic structure (see Figure 1-2).
Figure 1-2. Object resources are like jelly molds and you can use them to create any number of object
instances
Variables
A variable is a general programming term for something that can store information. In Game
Maker, variables can either store a number (for example, 3 , -12 , 151.33 ) or some text (for
example, demons don't like dragons ). You use variables in Game Maker to store all the unique
information about individual instances, such as their position on the screen, or their speed. So if
you create a new power variable in the Create event of a dragon object, then every dragon object
will have this variable, but each instance can have its own different value for power . It's also worth
noting that you must assign variables a value (using a Set Variable action, for example) before
you can use them (with a Test Variable action); otherwise, Game Maker will produce an error.