Paint quality depends on several factors, such as how it's blended and what type of resins are used. The better quality paints make use of enamel and urethane, while the older lacquer resins are lower quality.
Automotive paint is made up of a binder, pigment and carrier agent. The binder, or resin, is what defines the finish of the paint. There are three different types of resins available.
Lacquer paint is the cheapest to spray, but is illegal in many states. However, lacquer is often easier to spray than other alternatives.
Urethane and enamel based paints have glossier finishes. Although enamel is generally cheaper to buy, urethane offers a more durable finish.