Well, exponential (expo for short) is adjusted to tune the relationship between the throttle trigger posistion and the amount of power (or throw, if refering to servos that operate an engine) actually applied to the powerplant.
stock expo should be at 0%, also known as linear, meaning that when the trigger is pressed down half way, it will provide the car with half the availiable throttle.
Note: this adjustment can also be used for steering as well.

Any other adjustment besides linear turns this acceleration path into a progressive (or exponential) climb towards full throttle at a different rate compared to the relationship of the trigger posistion.

Simply put, negetive expo will give more low end control feel, yet it will have full throttle similar to an off/on switch after a certain trigger posistion, and the exact oppisite for positive expo, Linear is the happy medium that most people should stay with.

I don't know wtf panic breaking or steering is used for, perhaps it just smooths out rough, jerking control when applied.

Hope this made sense/helps.