Carbon dioxide (chemical formula: CO2) is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state.
Carbon dioxide is produced by all animals, plants, fungi and microorganisms during respiration and is used by plants during photosynthesis. In particular, it is produced by yeast as one of its byproducts as it consumes the sugars in the wort. (The other byproduct being alcohol.)
Carbon dioxide is also what produces the bubbles found in most beer. It gets there either from the yeast, in the case of bottle-conditioned beers, or is added artificially. The process of adding CO2 to a beverage is called carbonation.