Glycemic Index
Carbohydrates (carbs) are a key energy source for runners. The way carbs are used depends on the state you are in. If you are in the middle of exercise, these carbs tend to be used directly for energy. If you are at recovering from exercise, these carbs will go into quick access storage (Glycogen). If your Glycogen stores are full, then the carbs will tend to be stored as fat.
Some carbs are easily digested, with the fuel becoming ready for use quickly. These 'quick carbs' are great in the middle of a run, as the muscles (and brain) will start to burn them. If you are at rest, these quick carbs can raise the blood sugar levels quickly, causing a 'blood sugar spike'. The body reacts by producing insulin, which can overcompensate for the spike and result in a 'blood sugar crash'. Neither the spike nor the crash is good for you.
So what carbs are 'easily digested'? In some literature, quickly digested carbs are considered 'simple' and slow digesting carbs are 'complex', but this is not a useful division. The difference between simple and complex is based on the chemistry of the carb molecule - small molecules like sugar are 'simple' and big molecules like starch (bread, etc) are 'complex'. This division into simple and complex is unfortunately crap (biochemistry term meaning 'not useful').
The digestion of carbs is a sophisticated system that does not follow this simple division. Some simple carbs (Fructose) are very slow to digest, whereas some complex carbs (Maltodextrin) are very easy to digest. The actual measure of digestibility of carbs is normally called 'Glycemic Index' (GI), which is how much the blood sugar rises when a food is eaten [1]. For instance, white bread (a 'complex' carb, GI 70) has a higher GI than table sugar (a 'simple' carb, GI 60). This is because highly refined flour in bread is more easily digested than table sugar (which is half Fructose).
Understanding the GI of food is important to health. Spikes in blood sugar has been linked to Diabetes, heart disease and weight gain. As a runner, high GI food is great for taking in the middle of exercise or directly after. At other times, it's best to avoid high GI foods. The web site http://www.nutritiondata.com has a lot of nutritional information on many foods, and includes a 'glycemic load', which can be useful in choosing foods. The site http://www.glycemicindex.com/ has a database of GI values for food.
One of the factors limiting the use of GI in food labeling is that it has to be experimentally tested; it can't be measured based on the food due to the complexities of the human body. For instance sourdough bread has a lower GI than equivalent regular bread because the acidity in the sourdough bread slows digestion. Another example: the difference between white and most whole wheat bread is not significant.
Swapping high GI foods for low GI foods can be a very useful part of a Weight Loss program. A given number of calories of a high GI food will not keep us satiated as long as low GI. That means that eating a low GI food will stave off hunger for longer, causing us to eat less overall.
[1] Wikipedia - Glycemic Index
http://en.wikipedia.org/wiki/Glycemic_index
Recommended reading 'The New Glucose Revolution Complete Guide to Glycemic Index Values' http://www.amazon.com/gp/product/1569244782