CO2 (Carbon Dioxide) is a compound used in paintball in its gaseous state as a propellant. CO2 in the public's eye is a less stable propellant than HPA (high pressure air) and has been replaced by HPA because of its vulnerability to temperature changes.
The CO2 used in paintball is food grade, meaning it is as close to pure CO2 as possible. This is the same CO2 used in the brewing of beer and in home aquariums. CO2 tanks get more shots per fill than HPA, the reason for this is because of the fact that inside a CO2 tank there is liquid CO2. As the gaseous CO2 is used up there is more surface area for the liquid to expand into gas. Liquid CO2 however, is a double edged sword. The liquid CO2 can enter the paintball marker's system and cause damage to the internals of the marker as the liquid CO2 rapidly expands to a gas. CO2's phase change requires heat (endothermic), so CO2 is not very effective when used in the winter.
Shootdown is a loss of FPS (feet per second) over time that is usually caused by tank pressure. Shootdown occurs with CO2 when the CO2 fed marker is being fired very rapidly. This shootdown occurs because the CO2 tank has become too cold and there is little warm for the CO2 to use to expand into it's gaseous state.