This is an important and complicated point. "Simple" carbohydrates typicly refers to mono and dicsacharides such as glucose(mono) sucrose(di) fructose(mono) lactose(di) etc. "Complex" carbohydrates typicaly refers to carbohydrate polymers or strings in which a sugar such as glucose is esterified(linked together like beads on a necklace) many, many times. Glucoes which is alpha linked in this manner is what we call bread, rice, pasta etc. Glucose which is beta linked in this manner is what we call wood. Refining a complex carbohydrate typically breaks long pieces of carbohydrates into smaller pieces which increases the surface area available for enzymatic action thus increasing the rate of absorption and consequently blood sugar levles and a responsive insulin spike.
Fruit tends to be predominantly simple sugars and fiber but it tends to pack a low glycemic load,I explained glycemic load elsewhere in this thread, so fruit and low glycemic load foods tend to produce OVERALL less of a change in blood glucose and insulin levels. This is fairly relative from person to person and for me my tollerance for any type of dense carb source, be it fruit or potatos, is pretty low.
This is not quantum mechanics but it is somwhat complex and soundbite type information ALWAYS raises an exception which can cause a mountain of confusion. The Crossfit Journal dealing with glycemic index and nutrition flesh's out these concepts further as do the commonly refrenced books The Zone, Natural Hormonal Enhancement, Lights Out: Sleep, Sugar, and survival.
Once a firm grasp of these basic nutrition concepts is attained the ability to rationally modify ones diet is in sight.