The short answer would be that the cell membrane is a phospholipid bi-layer with embedded proteins. Cell membranes have two rows of phospholipids arranged so that the hydrophobic (water "hating") lipid chains are orientated towards each other, with the polar, hydrophilic (water "loving") heads outwards so that they are in contact with the environment inside and outside of the cell itself. The cell membrane acts as a barrier between the inside and outside of the cell, but various materials must move in and out for the cell to survive. Small, uncharged particles can pass straight through the lipid bilayer. Larger molecules move through channels made of protein. Membrane proteins are also involved in cell recognition. Cholesterol molecules are a part of cell membranes, helping regulate the degree of rigidity of the membrane.
In general, cell membranes are similar to each other, but the percentage of proteins can vary based on what type of cell or organelle it is enclosing. See the attached link for more information on this.