Fertilizers--The three main nutrients
The three numbers on packages of Fertilizers are there to tell you what percentage of the major nutrients are found in that particuliar Fertilizer.
Nitrogen(1st number)promotes plant growth. Lawn fertilizers have a higher concentration of nitrogen.
Phosphorus(2nd number)promotes root growth, blooming and production of fruit. When you are starting off plants you want to have a fertilizer high in Phosphorus so that the roots get a good start. Bloom Boosting fertilizers will also have a higher percentage of Phosphorus.
Potassium(3rd number)promotes hardiness in a plant. Percentage is higher in Winterizing fertilizers. Care should be taken as too much of this can "burn" plants.