JDM officially stands for Japanese domestic market. In simpler terms, it refers to the Japanese modded car scene. There are tons of differences between US and Japanese versions of cars, and even some cars that are only manufactured and legal to be driven in Japan. Some of these cars are actually completely illegal to own in the USA!
The main difference everyone first realizes between JDM and USA cars is the steering wheel location. In a JDM car, the steering wheel is on the right hand side of the cabin. This is one of the coolest aspects of owning a JDM vehicle. The JDM scene largely exists due to Japans car laws. In Japan it is incredibly hard to own a vehicle for an extended period of time. In fact, you can actually be penalized/subject to heavy fees for driving a car in Japan that is over a certain age or mile threshold.
This means people are often eager to get rid of their cars around 40,000-60,000 miles and buy new ones. These cars with 40,000 miles on them are still in perfectly good condition, which makes them so appealing to Americans.
Another interesting fact is that lots of JDM cars have built in speed limiters, which prevent them from going faster than ~110 miles per hour. This can obviously be removed when these cars are imported, but is still an interesting fact.
There are many websites out there with JDM in the title that often service JDM or import style vehicles, and the popularity of these classic JDM cars such as the GTR/Skyline is only growing!