Why does the avocado turn brown?

When the meat of the Avocado is exposed to air, oxidation occurs. This is because of an enzume called polyphenol oxidase. This enzyme works on phenolic compounds in the flesh of the avocado, changing their chemical structure and thus their color.
The best way to prevent oxygen intrusion is to take plastic wrap and seal it over the guacamole.
You can attack the other culprit, the enzyme, in a couple of ways. Refrigeration will slow down the action of the enzyme. Since the enzyme doesn't like acidic conditions, adding something acidic to the guacamole such as lime or lemon juice will slow the reaction of the enzyme with oxygen