The basic premise behind the spoilers, diffusers etc is to reduce the lift generated by your car.
Due to the shape of most cars, when moving, the air travelling above the car and underneath it, have different speeds. The air travelling over the top moves faster as it has to cover more distance, and the air travelling underneath your car travels slower relative to the air speed over your roof. this causes the top side of your car to have a lower pressure than the undercarrige, causing lift. your car essentially becomes a wing.
In order to reduce this effect, and produce downforce, people have come up with various solution.
The SPOILER works on the same principle, making the air that passes through it travel faster underneath it with respect to the air velocity on its top side. making it produce the opposite of lift, downforce.
Most, if not all spoilers have their effect at higher velocities. so your aerodynamics come into play at higher speeds. At lower speeds, you still rely on good old mechanical grip.
All the other little chin spoilers, underbody diffusers, rear diffusers in some way or form play on the above premice of varying air velocity along the shape of a car.
Throughout time, manufacturers like Mitsubishi, Ferrari, Subaru, Porsche have also come up with some "Out there" solutions like the vortex genarators and the good old air suckers on the F50 that also try to solve the good old downforce problem. Some more effective than others, but great marketing tool none the less.