Stunt Parks are generally the result of an admin or admins setting up one or many objects designed usually to battle against the forces of gravity and allow people to believe they can fly without an aircraft.
Who tends to organize Stunt Parks?
Generally any ingame admin might decide to do one, but don't specifically ask for them or you might just get whacked upside the head. They spawn a bunch of jumps, loops, and maybe the odd exploding barrels, and people from all over SA come to have some fun.
What things shouldn't I do when I am at the Stunt Park?
Admins might kick or kill you if you block access to the jumps or loops deliberately, or start DMing folk that are just out to have fun.
Can "normal" players start a Stunt Park?
Only when dealing with a natural stunt-able location, such as the airport jumps, Mount Chiliad, etc.