N.P.K. stands for the "N" Nitrogen, "P" Phosphores, "K" Potassium break down in fertilizer. The numbers should appear on all packages of fertilizer such as 20-20-20 or 18-24-12. (a common starter fertilizer)
Most fertilizers even though man made are made with simple elements and are totally safe. There are many fertilizers labled as "organic" and are actually the same thing as the other. It is a play on words because "organic" by the dictionary definition only means that it is carbon based.
Like the Bearswoman said using fish buried under each plant
is a good idea, and there are other natural fertilizers on the market as well. Personally I like using the man made stuff. It usually give better results and it is basically all the same stuff anyway.