WHY, WHAT AND HOW
All reactions are dependent on how much stuff you have. Stoichiometry helps you figure out how much of a compound you will need or maybe how much you started with. We want to take the time to explain that reactions are dependent upon the compounds involved and how much of each compound is needed. Stoichiometry can be used to find quantities such as the amount of products (in mass, moles, volume, etc.) that can be produced with given reactants and percent yield (the percentage of the given reactant that is made into the product). Stoichiometry calculations can predict how elements and components diluted in a standard solution react in experimental conditions. Stoichiometry is founded on the law of conservation of mass: the mass of the reactants equals the mass of the products. In stoichiometry, you base everything off the ratios of a chemical formula. These ratios are in moles.
ex.
O2 + 2 H2 > 2 H2O
The formula shows us that for every ONE O2 atom, there will be a need for TWO H2 atoms to react to make water. This is the basis of mole ratios. Stoichiometry provides the quantitative relationship between reactants and products in a chemical reaction. For example, when methane unites with oxygen in complete combustion.
What do you measure?
It could be anything. When you're doing problems in stoichiometry, you might look at...
 Mass of Reactants (chemicals before the reaction)
 Mass of Products (chemicals after the reaction)
 Chemical Equations
 Molecular Weights of Reactants and Products
 Formulas of Various Compounds
MAIN CALCULATION
Stoichiometry rests upon the very basic laws that help to understand it better, i.e., law of conservation of mass, the law of definite proportions (i.e., the law of constant composition) and the law of multiple proportions. In general, chemical reactions combine in definite ratios of chemicals. Since chemical reactions can neither create nor destroy matter, nor transmute one element into another, the amount of each element must be the same throughout the overall reaction. For example, the amount of element X on the reactant side must equal the amount of element X on the product side. Stoichiometry is often used to balance chemical equations. The term stoichiometry is also often used for the molar proportions of elements in stoichiometric compounds (composition stoichiometry).
Stoichiometry is not only used to balance chemical equations but also used in conversions, i.e., converting from grams to moles, or from grams to milliliters. For example, to find the number of moles in 2.00 g of NaCl, one would do the following:

CONVERTING GRAMS ← → MOLES ← → ATOMS ← → LITERS
MOLAR CALCULATIONS
Principle
All of stoichiometry can be solved relatively easily using dimensional analysis. Dimensional analysis is just using units, instead of numbers or variables, to do math, usually to see how they cancel out. For instance, it is easy to see that:
It is this principle that will guide you through solving most of the stoichiometry problems (chemical reaction problems) you will see in General Chemistry. Before you attempt to solve a problem, ask yourself: what do I have now? Where am I going? As long as you know how many (units) per (other units), this will make stoichiometry significantly easier.
Moles To mass
Calculating the mass of a sample from the number of moles it contains is quite simple. We use the molar mass (mass of one mole) of the substance to convert between mass and moles. When writing calculations, we denote the molar mass of a substance by an upper case "M" (e.g. M(Ne) means "the molar mass of neon"). As always, "n" stands for the number of moles and "m" indicates the mass of a substance.
Calculating Molar masses
Before we can do these types of calculations, we first have to know the molar mass. Fortunately, this is not difficult, as the molar mass is exactly the same as the atomic weight of an element. A table of atomic weights can be used to find the molar mass of elements (this information is often included in the periodic table).
WHAT MORE..???
However, there you don't typically measure the number of molecules before adding them into has it's characteristic proportions. The method of obtaining these from chemical formulas, equations, atomic weights and molecular weights, and determination of what and how much is used and produced in chemical processes, is the major concern of Stoichiometry.This is especially important in medicine, because we would want the amount of toxic side products formed from the biochemical processes acting on the consumed medicine in our body to be minimal. In a equation or a question the number of atoms you need will help you figure out how much of each substance you will need to make the reaction happen.
And to learn about more on this is much more new..!!!!