The Map is where the entire game takes place. The map is chosen at game start and never changes mid-game. OpenRA has countless community-created maps to choose from. (link?)
The map is divided into small cells. Cells are the game's distance units. Only one unit can stand on the same cell at any given time.
The map is not to be confused with the minimap.
Some non standard maps have custom rules that override the default behavior of "OpenRA - Red Alert". Custom rules can affect unit stats, build perquisites, etc.