Cell membranes are, at their most basic, composed of a phospholipid bilayer with some surface proteins embedded around the surface. Plasma membranes contain phospholipids, cholesterol, proteins and carbohydrates that are arrayed in regular, repeating rows to form a highly plastic surface for the cell.
Cell membranes are not solid structures. Instead, they maintain a consistency similar to that of vegetable oil and permit diffusion and cellular transport in ways that help maintain homeostasis within the cell. The building block of the plasma membrane is the phospholipid molecule. This molecule is shaped like a hairpin, with a hydrophilic phosphate head and hydrophobic lipid tails. These molecules always arrange themselves so their heads are oriented in the direction of water and their tails are not. This naturally produces a double layer of phospholipids with their tails interior to their heads. Cholesterol molecules, which are also hydrophobic, group themselves around the phospholipid tails.
Across both surfaces of the membrane, various proteins perform roles in keeping the cell functioning. Some adhere to the outer layer and signal other cells, some adhere to the inside and act as enzymes to speed up chemical reactions, and some pass through the membranes and act to transport molecules across the membrane into and out of the cell.