Nitrogen- Promotes growth and  chlorophyll formation.  All lawn fertilizers contain lots of nitrogen, but it's particularly prominent in spring/summer blends.  Most granules in the blend will release the nitrogen gradually but fast-release material is included, too.

A typical spring/summer blend has an NPK of 29-2-4

Phosphorus- Helps establish roots in grass grown from seed and develops roots on grass plugs and sod.  Buy low- or no-phosphorus fertilizer if you're not establishing seed, plugs or sod- the runoff harms waterways by promoting aquatic weed growth.

Seed starter blends have a phorphorus heave NPK of 18-24-6

Potassium- Contributes to overall plant health, stress resistance and heat and cold tolerance.  Combined with nitrogen, potassium helps the plants form carbohydrates over the winter.  High potassium fall fertilizers yield lawn health benefits all year long.

Fall/early winter blends typically have an NPK of 22-2-14

Information courtesy of Popular Mechanics April 2008