What is the difference between a calorie and a food calorie?

Calories can be broadly classified as

1.large calories

2.small calories

1. Large calories : This is also called as kilogram calorie, dietary calorie, nutritionist's calorie or food calorie.

"It is the energy required to increase the temperature of 1 kg of water by 1 degree C under 1 atmospheric pressure."

In conversion, 1kcal = 1 cal (in terms of large cal)

2. Small Calories : This is also called as gram calorie. (the other term you are refering in the question, 'calorie')

"It is the energy required to raise the temperature of 1gm of water by 1 degree C under 1 atmospheric pressure"

In conversion, 1Kcal = 1000 cal (interms of small cal)

So, when you are talking about 'calories', it refers to small calories.

And when you are talking about food calories or calories related to dietary facts, it refers to 'large calories', and remember the conversion holds.