Cellular automata (CA) are systems consisting of the following:

• A cellular space or otherwise discrete division of the world into independent entities; usually in GIS, this is assumed to be the cells of a raster grid plus the properties of the boundaries of the space.
• An exhaustive set of states into which each of the cells falls, such as urban/nonurban, or land use classes.
• An initial or starting configuration of the states over the space, often a map of a spatial distribution at some initial time period.
• A neighborhood for a cell, most commonly the cell's immediate neighboring cells.
• A set of rules that determines how states behave during a single time step.

Time in the system then takes the form of an ongoing series of steps. The model begins with ...