Nitrile and neoprene are types of synthetic rubber, but have different chemical structures. The two rubbers vary in resistance to heat, abrasion, flame, petroleum or weather, so are used for different applications.
Nitrile is a copolymer, it is made up of a mixture of two molecules. These are butadiene and acrylonitrile. Neoprene is a homopolymer, made of a single substance called chlorobutadiene. The proportion of butadiene and acrylonitrile can be adjusted in nitrile rubber for specific purposes. For example, increasing the proportion of acrylonitrile, increase the rubber's resistance to heat.
Nitrile rubber is extremely resistant to petroleum products and so is the most commonly used rubber in making seals, such as those for cars. Neoprene is not as resistant to petroleum products as nitrile rubber but it resists weather better than nitrile rubber. It is also more resistant to refrigerants than nitrile so it is used in air conditioning systems.
Both rubbers are used for seals in industrial and automotive machinery, but neoprene is more expensive than nitrile rubber, so it is generally used in cases where resistance to oil, heat, flame and abrasion is necessary.