Net carbs represent the carbs that affect your blood sugar and are therefore counted. They are calculated thusly:
NET Carbs = Total Carbs - Fiber grams- (sugar alcohols*)
Fiber is a type of carbohydrate (see Ch. 8 DANDR 2002 for a complete explanation of fiber) which has little effect on blood sugar, so it is subtracted from the Total carb amount.
Sugar alcohols, or polyols, are a controversial topic in low carbing. They are artificial sweeteners that mimic the baking qualities of sugar. They supposedly have little effect on the blood sugar too, which leads some low carb product manufacturers to subtract these substances from the Total carbs. However, some low carbers experience weight loss stalls or slow downs when using these products. Not to mention sugar alcohols are notorious for causing diarrhea, abdominal cramping, and gassy discomfort.
Due to the effects on weight loss, some low carbers will either subtract