the difference is mainly in how quickly your body will convert the carbohydrate molecules to glucose, i.e. the stuff that gets stored in muscles as fuel for work (lifting). Basically, and although this is a bit of a dated explanation, 'simple' carbs get converted into glucose at a much higher rate than 'complex' carbs, and are thus more valuable either during, or right after a workout. it basically comes down to the number of chemical bonds linking the molecules... the more unrefined the carbohydrate is, the more the body needs to work in order to digest it. Also, the quicker the carbs are converted into glucose, along with the amount of carbs ingested, the stronger the insulin spike will be. This basically means that if youre consistently downing simple carbs (candy, soda, white rice, rice cakes, cereal, etc.) your body will be consistently pumping insulin... creating a primarily anabolic environment within your body. this is great for bulking, but if you're trying to lean out, not so much. also, your body can only store so much glycogen before the insulin starts shuttling glucose into fat cells, so make sure your workouts are intense, and match carb intake to your work capacity each day. fats and protein should stay consistent.
one last thing. John Berardi is real big on nutrient timing... personally, i think the guy's a genius. if ur looking for a generally fit looking body, keep carb intake to only fruits and veggies until right after your workout, then down some sugar (dextrose ideally, but any really simple, fast digesting carb will work) and fast digesting whey isolate (2:1 ratio) and then down a protein and starch (pasta, oats, rice, bread etc.) meal within two hours of your workout. hope this helped...