Nitric oxide (NO), also known as nitrosyl radical, is a free radical that is an important signaling molecule.  For example, NO causes smooth muscles in blood vessels to relax, thereby resulting in vasodilation and increased blood flow through the blood vessel.  These effects are limited to small biological regions since NO is highly reactive with a lifetime of a few seconds and is quickly metabolized in the body. 
Typically, NO gas is supplied in a bottled gaseous form diluted in nitrogen gas (N2).  Great care has to be taken to prevent the presence of even trace amounts of oxygen (O2) in the tank of NO gas because NO, in the presence of O2, is oxidized into nitrogen dioxide (NO2).  Unlike NO, the part per million levels of NO2 gas is highly toxic if inhaled and can form nitric and nitrous acid in the lungs.