The cell membrane, also known as the plasma membrane, is a thin, semi-permeable layer that surrounds the cell. It separates the cell's internal environment from the external environment and regulates the movement of substances in and out of the cell. The cell membrane is composed of a phospholipid bilayer with embedded proteins, cholesterol, and other molecules. It is located at the outermost part of the cell and is essential for maintaining the cell's shape, protecting it from the external environment, and allowing communication with other cells.