Key difference is total carbs includes all the different types of carb in a food (starches, dietary fiber, and sugars). Net carbs only include carbs that the body can fully digest into glucose.
When calculating net carbs, dietary fiber and sugar alcohols (if applicable) may be subtracted from the amount of total carbohydrates.
Net Carbs = Total Carbohydrates - Fiber - Sugar Alcohols (if applicable)