It often is -- when yeast ferments a substance, it releases CO2. During the fermentation of beer (or again most anything, but I'll just use beer as a shorthand)

It often is -- when yeast ferments a substance, it releases CO2. During the fermentation of beer (or again most anything, but I'll just use beer as a shorthand), that CO2 is released through an airlock. When the beer is bottled, any beer made in large quantities will be sterilized first with potassium metabisulfite (I think) so that fermentation doesn't continue in the can, bottle, or keg. If you don't kill the yeast, or if you bottle your beer with an extra batch of yeast (called bottling yeast) and sugar, you'll get fermentation continuing in the bottle, which will result in the CO2 produced being absorbed into the liquid. Obviously this is not as precise and will create different flavors and mouthfeel as a beer ages. If you do kill the yeast but still want fizz, you just force carb your beer with CO2 from a container.