A basic idea that you must know is that materials soluble in water are not soluble in lipids, and materials soluble in lipids are not soluble in water. The bulk of the membrane is composed of lipids - phospholipids and cholesterol. Thus, lipid soluble molecules are also able to freely diffuse across the membrane.  In addition, small water soluble ions and molecules (including water!) are able to diffuse across the cell membrane through pores or channels in protein molecules.   However, large water  soluble molecules such as proteins and nucleic acid are not lipid soluble and hence can't freely diffuse across the membrane.