The lipids we'll be studying are phospholipids, the primary components of cellular membranes. Phospholipids are amphipathic; that is, they are both hydrophilic and hydrophobic. The "head" of a lipid moleule is negatively charged phosphate group and the two "tails" are highly hydrophobic hydrocarbon chains.
Phospholipid tails will congregate together to form a local hydrophobic environment. This leaves the charged phosphate groups facing out into the hydrophilic environment. There are three structures that phospholipids can form because of their ampiphatic nature. Each represents a phospholipid.
A phospholipid bilayer is approximately 5 nm thick. This membrane is semipermeable, meaning that most molecules are excluded but some molecules are allowed to pass freely (diffuse) through the membrane.