Foods with a high GI, will boost your energy short term, but will end in a low.
And so foods with a low GI will help to give you more long term energy.
Your friends are misinformed. Bad carbs would be most processed wheats and sugars. Candy, most all baked goods, white rice, and white bread being the most obvious.
White potatoes are bad, sweet potatoes are better.
Low GI, complex carb based foods are good for energy levels: oats and other grains, whole wheat bread, carrots and corn (both in moderation) and other such whole foods.
As I said before, Google it. You should read up on it. and find a list of the GI numbers of all foods, you wil be very suprised.
Its not so much that low GI is good and high GI is bad, they both have their uses and will both make you fat if you eat too much.
Low GI foods just burn slower, but have the same amount of calories as hi GI foods.