Fertilizers come in a different array of formulations. They can be divided into 5 categories: growth, balanced, starter, complete and special need. These are classified based on the amounts of nitrogen, phosphorous and potassium present in the bag or the N-P-K ratio.
Ammonium nitrate is a fertilizer with the N-P-K designation 34-0-0 with chemical formula NH4NO3. It is the highest Nitrogen fertilizer you can buy which contains 34% Nitrogen and the other 66% or 67% is inert material.
Nitrogen helps plant foliage to grow strong. Phosphorous helps roots and flowers grow and develop while Potassium is important for overall plant health.
The Journal Times answered a query from a letter regarding a list of preferred fertilizers for different types of plants. Read it here.