A cell membrane has several key features that are present throughout all cells with a membrane. First of all, most cell membranes are composed of two layers of lipids or fats as illustrated in the picture above. Each layer is composed of individual lipid molecules that have two distinct regions. The head of the lipid likes to be near water and the tail region does not like water. As a result of this disposition toward water the lipid molecules pack to form an external layer with their head toward the exterior and their tails within the two layers of the membrane.
Cellular membranes are fluid, which means that the lipids are able to move within the layers.
Another important component of membranes, that are also able to move within the lipid layer and between layers is proteins. These proteins give the membrane much of its ability to selectively allow some substances out while allowing others in. Proteins can act as transport molecules which actively carry molecules or ions across the membrane, or they can simply provide a channel for molecules of a certain size to pass freely through.