The glycaemic index measures the speed with which glucose in foods is released into the body and absorbed by the bloodstream (in other words, its ability to increase blood sugar levels).
Foods are ranked on a scale from 0 - 100, with glucose, the most hyperglycaemic sugar, at 100.
The nearer a food's value is to 100 on the scale, the faster it is at releasing glucose. The nearer it is to 0, the slower it is.
Since carbohydrates are the only foods that affect blood sugar levels during their metabolisation, they are the only group that have a glycaemic index.