Phospholipids are lipids with a phosphate functional group (PO_4-3) attached at the end. Lipids are a group of molecules that include fats, fatty acids, and waxes that occur naturally in biological systems.

Phospholipids are important because they form cell membranes. When dissolved in water, phospholipids will form a bilayer structure because the hydrophobic tails will face each other to avoid the water, and the hydrophilic phosphate groups will face the water.

Hydrophobic – repelled by water
Hydrophilic – attracted to water

The figure shows how phospholipids form membrane structures.
phospholipids*

Phospholipid bilayer membranes form the boundaries of our cells. Without them, our bodies would have no way to keep all of our cell contents separate and could not function.

*The figure is from: http://en.wikipedia.org/wiki/Phospholipid