I think it helps to have a basic understanding of how alcohol home brewing kits, so I am including this diagram of the process with an explanation below:
After making the wort, you transfer it to a carboy where fermentation will take place. The carboy for home brewing kits are made out of plastic or glass; I plan on doing an article on the advantages/disadvantages of each at a later date. After transferring your wort, the next step is to pitch the yeast. The yeast feeds on the sugar in the wort, and the byproducts of this process are CO2 and Alcohol.
In this particular example the CO2 is actually released through an airlock at the top of the carboy. The airlock allows the CO2 to be released, while preventing oxygen from entering the carboy and contaminating/oxidizing your brew. The wort stays in the carboy for about 2 weeks or until fermentation is complete (when you stop seeing bubbles coming out of the airlock on a regular basis.