There are all kinds of carbohydrates. There are big ones and teeny tiny ones, and everything in between! In terms of carbohydrate digestion, simply put, only monosaccharides are absorbed from the gastrointestinal tract. This means that other, larger sugars like olestraTM are not absorbed.
The human can break down most carbohydrates to mono-saccharides, but not those with β-1,4 linkages. The foods that have β-1,4 linkages can essentially be considered roughage -- inert matter that passes through the system undigested. Roughage is very good in keeping people regular, like any consumer of Raisin Bran will tell you.
Those foods that have carbohydrates with lots of linkages take longer to be digested, and so usually produce a slower peak in the person consuming them. In contrast, those foods that have fewer or no linkages (aka simple sugars) are consumed more readily, and produce higher peaks more quickly. This, my friends, is what people call glycemic index.