The cell membrane is perhaps the most important part of a cell, because without it there would be nothing to hold the cell's contents. It is mostly made of a double layer of lipids, or a lipid bilayer. The rest of the membrane is composed of various proteins and carbohydrates. The proteins may act as channels for certain molecules to pass through, while others act as pumps and can force molecules through. Many of the carbohydrates help to identify cells so they are able to interact with each other.
The cellular membrane is very important in the process of phagocytosis. When the macrophage engulfs a bacterium, it's cellular membrane envelopes the other cell, and the pocket breaks away within the cell and forms a vacuole, also known as a phagosome.