A calorie is the amount of energy needed to raise 1mL of water 1 degrees celsius.
A Calorie (also called Kilocalorie) is the amount of energy needed to raise 1L of water 1 degrees celsius.
A kilocalorie is what is used to determine the Calorie content of foods.
In layman's terms, A kcal is the same thing as a Calorie. The can of seafood that has 120kcal has 120 Calories.