Stoichiometry

Stiochio --> element
Metry----> measurment

Therefore stoichiometry is the theoretical measurement of elements.
Now what is that supposed to mean?
Well, we can use stoichiometry to determine the amount of product a reaction will produce and vice versa.

First of all we need to look at the chemical equation we are using.
Eg. NH₃ + O₂ à H₂O + NO

Which now needs to be balanced correctly
Eg. 4NH₃ + 5O₂ à 6H₂O + 4NO

The coefficients serve as the mole ratio.
This means that when there are 4 moles of NH₃ and 5 moles of NO they will 6 moles of water and 4 moles of NO.

Now the fun part starts
Generally you will be given some amount of one of the reactants/products and have to figure out how much of  another molecule it will produce.
Using the mole ratios you multiply the amount you have by a fraction with what you are trying to find out on the top and the amount you already have on the bottom. This will give you the number of moles you are trying to figure out.

Eg. If you have 4.3 moles of NH₃ how many moles of H₂O will be produced?

4.3 Mol NH₃ x 6 Mol H₂O   = 6.4 Mol of H₂O
                        4 Mol NH₃
 Here is another example


This website is an excellent resource with tutorials and videos
http://www.chemcollective.org/stoich/reaction_stoi.php

Another example is provided in this video
http://www.khanacademy.org/video/stoichiometry-example-problem-1?playlist=Chemistry