Like Fregus says.
Sugar is a carbohydrate, so is flour. starch and so on, sugar is a very quickly absorbed carb so will cause spikes very easily.
Some more complex carbs are more slowly absorbed as the body has to change them into sugar first (that's the simple explanation!).
So sugars are a bad (for us) form of carbs, potatoes, cereals, some fruits are also bad but not quite as bad as sugar. Then there are small amounts of carbs in things like vegetables, protien etc and they are the "not so bad carbs".
It also depends on how you eat stuff (for instance, a couple of small potatoes eaten with a lot of cabbage is OK for me) and how much you eat. I can eat a level teaspoon of normal jam on my home made bread toast without any spikes. I cannot eat a small bowl of porridge with or without sugar.
We all vary, so you just have to test until you kmow what you can and cannot eat.
H