A calorie is the unit of energy needed to heat a litre of water by one degree.
"Calorie" (with a capital "C") is common notation for a "kilocalorie" which equals 1000 calories (with a lowercase "c"). Since "calories" are too small a unit of measurement, nutrition labels give...
0.001
1000 calories + 1 kilocalorie
1000 kcal = 1 cal