A window is a basic element of the gameplay that allows zombies to enter the map, or block them from entering, your playing area on most conventional maps. A window can consist of one or two doors, though they cannot be opened or closed. Behind doors, usally holds a spawner for zombies. Zombies approach a window and will break the wooden boards covering it and walk thorugh the door, passing through it. It is also possible for a zombie to deal damage to you through the boards and vice-versa for the player. When a player approaches a window, they will be pushed back, not allowing them to get past it.
It is possible to repair the window, keeping the zombies breaking it at bay. You can repair only until there are 5 visable boards on the window. Depending your difficulity, you can gain a certain amount of points per board replaced.