It's a bit of a carb minefield isn't it!
When it comes to diabetes, it's a common misconception that 'you can't eat sugar' i.e cakes and chocolate but sugar comes in many forms.
In simple terms, sugar is the body's fuel. The body has to break down sugars into glucose (the simplest form of sugar) before it can absorb it where insulin then converts it into energy.
If you imagine 'sugars' as a bead necklace, the body can only absorb them when they are one bead long: Glucose is one bead long (a monosaccharide).
Other more complex sugars are several beads long (disaccharides and polysaccharides). Carbohydrates (starches) are complex sugars with varying lengths of 'beads' - nothing more. Each 'chain' has to be broken down to one bead (glucose) before it can be absorbed.
So when you are a diabetic and eat lots of carbs, your blood sugar levels rise because carbs ARE sugar.
And if those carbs are refined (white bread, pasta, rice etc) then the blood sugar levels rise FAST and can go quite high as the body is flooded with digested sugar looking for insulin to convert it to energy. Unfortunately for a type 2 diabetic, there is a distinct lack of good quality insulin around to do that converting! (and in type 1s, there's none at all!)
That's why it's not just the 'obvious' sugars like sweets and cakes that are diabetic 'baddies' but large quantities of starchy carbs are too. Same for fruit with its fructose.
Wholewheat cereals, oatmeal and other low GI foods take longer for the body to digest and give it a fighting chance to metabolise the sugars slowly as they're drip-fed through, avoiding the high blood sugars found in type 2 diabetics after a meal heavy in refined carbs.
Hope this sheds a bit of light on what a carb is (although it's still very confusing as to WHAT contains good or bad carbs). I mean, who would have thought a tomato or melon is carby?