REST between sets and AB QUESTIONS! (end)

[Need-help!!]

Ok, straight up, what is the benefit of less rest between sets and what is the benefit of more rest between sets. I know there is a certain point where the muscle is ready to work again. However, does that time increase as you progress (i.e. getting stronger, lifting more, more intensity, more rest) or decrease (i.e. muscles developing muscle stamina and don't need as much rest between sets to recover). Is there really such a thing as lifting for definition? In the extreme since, what are the benefits of giant sets (sets in tandem) - ESPECIALLY in regard to the abs. I know that body builders start decreasing rest and increasing rep range in their workouts - WHY? I don't think it burns more fat, they're working fatigued muscles so they maybe they don't wanna spend as much time in the gym, yet still get that burn w/o building much new muscle. To not digress, WHAT REST SHOULD I HAVE BETWEEN AB SETS. 30-40 sec, 1 min, or no rest at all (no rest = working fatigued muscles). I AM SO CONFUSED! WHAT SHOULD MY REP RANGE BE? SHOULD I BE CHANGING THE RANGE AND REST TO PREVENT the AB MUSCLES FROM PLEATAUing? Weights on obliques - yes or no.

ANy help would be greatly appreciated, I'm SOOOO Lost!