The glycemic index, or GI, is a measure of how a given food affects
blood glucose (bG). Some complex carbohydrates affect bG much more
drastically than others. Some, such as white bread, affect bG even more
than sugar (sucrose).

This was quite a surprise when the research was first published in 1981.
It really should not have been such a surprise. "Sugar", meaning
sucrose, decomposes in the gut to equal parts of glucose and fructose.
Fructose, as expected, has only a small effect on bG. Even
professionals, it turns out, were swayed in their thinking by the evil
charm of the word "sugar" and failed to take into account the
differences among the many kinds of sugar found in foods.

To use the glycemic index in a real-life diet, you must combine the GI
of various foods using a weighted average. Rick Mendosa's article (see
below) has information on simple calculations for mixed meals, which
recent research has shown to be reliable.

It remains difficult to predict the GI of high fat meals because of the
multiple affects of the fat, especially the way it slows the gut. For
example, a baked potato has a very high GI (one of the famous,
unexpected examples), but adding butter to it lowers the GI greatly.
This is a good reason to reduce dietary fat (if you needed another
reason), since doing so makes the effect of carbohydrates more
predictable.

If you don't want to go to the effort of full GI calculations, the
important thing is to understand that foods may affect your bG profile
in ways that you wouldn't expect from categorizations such as "simple
sugar" and "complex carbohydrate". Build your knowledge about your own
response to different foods and meals by monitoring and keeping
records, and avoid assumptions.

Rick Mendosa <mendosa(AT)mendosa.com> has written an excellent and thorough
article about the glycemic index. He also  maintains a glycemic index
list. I highly recommend that you check out

   http://www.mendosa.com/gi.htm

[Thanks to Rick for information he provided for this section.]