What is the difference between complex carbohydrates and simple carbohydrates?

The terms simple and complex carbohydrates were originally coined to differentiate between sugary and starchy foods, based on the chemical structure of the carbohydrate they contained. Sugars were simple and starches were complex, simply because sugars were small molecules and starches were big. By virtue of their large size, it was assumed that complex carbohydrates were more slowly digested and absorbed, and caused a small and gradual rise in blood glucose levels. Sugars, on the other hand, were assumed to be digested rapidly, causing a large rise in blood glucose levels.

Complex carbohydrates are also used to describe starchy foods such as potatoes, bread, rice, pasta, legumes, and breakfast cereals. Simple carbohydrates refers to sugars, as found in soft drinks, candy, ice-cream, cookies, cakes, and fruit.