Yes.  It is a way of looking at food in terms of how it affects your
blood sugar.  One of the nice things about this "diet" (rather, point of
view) is that it gets closer to the concept of eating to fuel your body,
i.e., choosing foods that your body can use well rather than eating just
for entertainment or for a quick energy rush.

There are a variety of books on this, but I'd google for it if I were
you.  Also be aware that the GI concept underlies quite a few of the
named diets: it's the basis for the Zone Diets (by Barry Sears, I
recommend those as quite readable) and behind the South Beach Diet and
really the basis in my opinion of a lot of the Atkins stuff if you
really understand why Atkins works inside your body (hint: it's not
ketosis.)

One reason to pay attention to the glycemic index is that it can help
you manage hunger better on a small calorie budget.  But the REAL reason
to pay attention to it is that the high glycemic foods tend to be crappy
foods.  Rice Krispies, for example, are pretty indistinguishable from
pure table sugar to your body.  Understanding how that works is
important to conquering your eating woes.

Dally

Basically avoiding certain carbohydrate foods (mostly sugar-rich, but
also some highly-processed ones) that are alleged to raise one's blood
sugar too fast, or too slowly, or something, so allegedly making one
feel hungrier sooner after them.

The GI (Glycemic Index) diet is about choosing food, based it's it
affect on your blood sugar.  Basically, it's the 'foundation' on which
most low carb diets are based (Atkins, South Beach, Zone etc.)

It's more popular in Europe, and in Australia the GI information is
part of the nutrition labelling.

It was generally considered 'too complicated' for the American market,
and so Atkins is like 'The GI Diet for Dummies' .  No offense meant at
to my American friends (I love ya to death)... by 'for dummies' I mean
simplified ala the 'For Dummies' series of books.

I think The Zone most closely approximates the GI Diet and deals with
how combining foods affect blood sugar differently than when eating a
particular food individually...  though 'The Zone' is also considered
'too complicated' by many.

In the opinions of some (I'm not commenting personally) Atkins is a
'GI Diet' gone wrong, oversimplified too much, with the result that it
excludes many good foods that would be allowed on GI, and allows many
'bad' foods simply because they are low in carbs.  On the other hand,
it's (Atkins) easy to follow - and what good is a diet that isn't
followed?  Advantage Atkins: easy to follow.  Advantage GI: less
restrictive.

Jeanie Miller-Brand is one of the leading expert on the GI diet.

There have been a lot of good answers to this question already. For a
listing of glycemic indexes of various foods, you might want to look at
http://www.glycemicindex.com/.  There is also a decent article about the
various popular diet plans at
http://www.cspinet.org/nah/01_04/cover.pdf.  This article include an
interview with Thomas Wolever and an explanation of glycemic index.
Here's an excerpt from the article:

"But unlike the other books, The New Glucose Revolution was written by
scientists who have actually studied the glycemic index. In fact, one of
its authors, Thomas Wolever of the University of Toronto, was one of the
researchers who helped devise the scale.

"The glycemic index is no magic bullet for dieters, says Wolever.
“I’ve yet to see evidence that a low-GI diet aids weight loss,” he
explains.  “One or two studies show it and a number of others don’t.”