Nitrox is a mix of nitrogen and oxygen. Usually nitrox contains more oxygen than air (>21%) and in such case it's called Enriched Air Nitrox or EAN. Some call any mixture of these two gases nitrox. For recreational diving a blend of up to 40% of oxygen is used, however technical divers use hotter (containig >40% oxygen) nitroxes to accelerate decompression. The two most popular mixes are EANx32 and EANx36 containing respectively 32% and 36% of oxygen.
Nitrox diving has become popular because because of the advantages this breathing mix offers. The main advantage is that because of lower nitrogen level divers body absorbs less nitrogen and thus the No Decompression Limits (NDLs) are longer. That means one can spend more time underwater or increase the seafty margin by using nitrox and staying withing air NDLs. Some people also report feeling less tired after a nitrox dive compared to an air dive. Previously people thought that breathing nitrox reduces effects of nitrogen narosis, however this turned out to be untrue because oxygen's narcotic potential is even higher:)
There are some dangers when it comes to diving with EAN and these are: oxygen toxicity and risk of fire/explosion. Both of these come from the increased level of oxygen in the mix and are easily avoided.
Fire may be caused when oxygen rich breathing mix comes into contact with materials that are not oxygen compatible or not oxygen clean, meaning that they simply start to burn in contact with such mix. To avoid this simply use oxygen clean and compatible equipment when diving with nitrox.
There are two types of oxygen toxicity: central nervous system oxygen toxicity (CNS) and pulmonary oxygen toxicity. The cause of CNS is still unknown, it is observed when the partial pressure of oxygen in the breathing mix is above 1.4 ata. CNS may result in convulsions (the most severe case), which may be lethal to a diver. Pulmonary oxygen toxicity is caused by prolonged exposure to higher than normal oxygen levels and results in lung inflamantion, coughing and decreased lung volume. These symptoms usually recede. To avoid oxygen toxicity you must monitor your oxygen exposure and maximum operating depth (MOD). MOD is the depth at which oxygen partial pressure reaches the maximum safe level (1.4 ata). Modern diving computers can usually track both of these parameters and alert the diver when he is reaching the limits. This feature does not mean you don't have to plan your dive.
Diving cylinders used with nitrox should be specially marked and have the MOD for the mix that they contain written on them. To mark cylinders usually a sticker is used, just like the one below:
EAN can be obtained by different methods. Generally you can add some oxygen to air or remove some nitrogen from it. Adding oxygen is done in partial pressure mixing or continuous blending mixing and oxygen removal is performed by using a special membrane in gas separation mixing
Wikipedia has a fairly good article on gas blending
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