A cell wall is the wall of a cell in plants, bacteria, fungi, algae, and some archaea. Animal cells do not have cell walls, nor do protozoa. Cell walls protect the cells from damage. It is also there to make the cell strong, to keep its shape, and to control the growing of the cell and plant.
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is outside the cell membrane and gives these cells support and protection, as well as acting as a filter. The cell wall also acts as a pressure vessel, preventing over-expansion when water enters the cell by osmosis.
The material in the cell wall varies. In plants and algae, the cell wall is made of long molecules of cellulose, pectin and hemicellulose. The cell wall has channels which let some proteins in and keeps others out. Water and small molecules can go through the cell wall and the cell membrane.