+Expo increases the speed of the action and -Expo slows down the particular action.

Look down at the wheels during turning, and it will become pretty obvious whichever radio you use.

½ correct. While it slows down at first, or speed up at first, it will get exponentially slower or faster at max servo travel to counter the slow or fast at first.

Isn't it also possible to use expo to adjust over-steer/under-steer?

From what you're saying here, i'm getting this.
A positive exponential would give me a "speed up" at first (basically the servo directs its power at that one spot) but then smooths to max servo travel. If i were to use that on a throttle servo for a nitro engine, i would get my power surge right at the early stages of liftoff like an electric and then it would make its way up to the max.

Negative exponential is completely the opposite. Since you said smoother, the throttle output would take its time to get the car moving first and then reach up to its max travel.

Does this sound somewhat correct?

That is how all the radios I have used are setup.
It is usually used only to the steering of the car though, as the throttle is often kept linear.

Most nitro engines have their power up at the top of the range, so adding positive exponential would get you up to that power faster, but it would decrease your slow speed engine control. I would just keep it linear, or add just a tiny amount of expo.

I have always thought the easiest way to explain exponential is how sensitive the control is around neutral....+ for more sensitive and - for less. And again, Spektrum is opposite.

I'm not sure why you just dont try it and see what the differences are....thats the best way to understand this.

I did see some differences when i tried it for the first time. I just wanted to know how these differences can benefit me on the track and such. But i should good for now, i'll experiment some more when i actually run it on a track. Thanks for everything guys!

Not really. It changes the amount of the radio inputs, it doesn't change how the chassis handles or corners. Some might suggest it does, but it's really only masking the chassis handling to an extent. If a car has over steer, it may not be so apparent if the wheel isn't turned much. But turn the wheel enough and it will still over steer, expo won't change or stop it.