Membrane serves an essential function of choosing to let all the necessary molecules in and wastes out. It is composed of two layers of phospholipids as well as proteins, carbohydrates and lipids. Phospholipids move freely on the membrane giving it the "fluid" term. Different proteins are embedded into the membrane in a nonuniform fashion. Therefore, membrane looks like a mosaic.
It's worth noting that the upper and bottom leaflet of membrane are not identical in their composition.