Up until now, we've tended to consider simple carbohydrates, present in sweet-tasting foods (sweets, fizzy drinks etc.), as "fast" and complex carbohydrates, present in foods containing starch (bread, pasta, rice, cereals...) as "slow".
The former were deemed to produce a fleeting peak in blood sugar levels, and the latter were said to 'fill you up' and deliver long-lasting energy.
However, when GI first came in, this classification was deemed too simplistic - it's not necessarily the simple carbohydrates that are the quickest. So bread and white rice, which are rich in starch and therefore considered slow, actually have a high glycemic index. Naturally, in an age when obesity and conditions like diabetes are rife, this discovery shook up the way we think about carbs.