Taken from kjjm4 off PBNCompressed air is just what it sounds like; it is the atmospheric air we breathe, compressed to high pressure. N2 is nitrogen. Over half of the air we breathe is nitrogen. When used to power a paintball marker, they perform the same. Most places fill with HPA.
The advantage of air/N2 over CO2 is that CO2 is stored in the bottle as a liquid, and must expand to a gas to power most paintball guns. This process makes the air unstable. Also, the expansion process absorbs heat, which is why your gun gets cold when you shoot with CO2. Because of this, CO2 will cause you velocity to spike at times. CO2 has problem in certain weather also. Electronic guns have problems with the electronics freezing, thus causing problems. Some problems are severe.
HPA is always in a gaseous form, thus making it more stable and more consistent when powering a paintball marker.