The Plasma Membrane Page
This is indeed the last linked section in that list. But if you didn't go in order, then you probably really don't care. Note: When I say 'plasma membrane', I'm not referring to the whole entire membrane, as in the membrane you see in the top picture on the main page. This is talking about just the bilipid layer, by itself, without all the proteins and cholesterol and all that jazz. Okay, here it is.
On the Phospholipid Page (if you haven't already read it, I really suggest you go do that) the basic structure of a single phospholipid is explained. The bilipid layer is, obviously, made up of two layers of phospholipids. They are arranged so that the heads are facing outward, and the tails are facing inward, towards each other. This happens because of polarity. On both the inside and the outside of the membrane, there is a high concentration of water. Water is a polar molecule. The heads of the phospholipids are also polar, and so they are attracted towards the water molecules. This makes them kind of "snap" into formation, pointing towards the water molecules. Because the tails are nonpolar and aren't attracted to anything, they're stuck in the middle between the polar heads. It is because of the differences in polarity that ions (charged) can't diffuse through the membrane, and have to use facilitated diffusion. The picture below that I found is a very good example of how the phospholipids are arranged. The light blue ("aqueous") is the water; the darker blue parts are the polar heads; and the very orange middle part is where the nonpolar tails are located.
Diagram of a Bilipid Layer
Citation
Picture source
