When I started my professional career, I was never really sure what the difference is between Carbon Dioxide and Carbon Monoxide and got a bit confused which is doing what.
First of all the major difference is that Carbon Dioxide (CO₂) is a by-product when the body burns energy. To be able to do this your body uses oxygen and fuel. It is also a Greenhouse gas, too much in the atmosphere causes temperatures to raise. And finally we use it for carbonated drinks like Coke, Sprite and so on.
Carbon monoxide (CO) is man made and is produced by the incomplete combustion of carbon containing compounds like exhaust smoke or cigarettes. This is most of the time causing bad air, which can be fatal for divers.