The calorie comes from the metric system and it's very useful to control the energy intake. This has several benefits of main importance because when an excess appears, the organism accumulates energy principally in the form of adipose tissue. A bit of fat doesn't worry but above a certain limit it drives to multiple health risks thus it should be rationally controlled using the tools that the development provides.
First of all there are different metabolisms related to the age, with young people having less probabilities of accumulating energy. In adulthood the metabolism changes and it performs a better overall utilization, so the necessary quantity to maintain the functions is lower.
Normally in the packaging of food the user can find nutritional information about calories. Anything with grease must have a high number since this kind of matter is one of the most important talking about energy density. For giving you an idea, coal or TNT don't rival fat.
Another aspect of relevance is the extreme optimization that inherently any human organism has, that outperforms any sort of local effort to compare. Even running long distances the consumption ends being scarce and then, how could you expect to lose those high number of calories?
It's at all an easy feat... and any individual must remember that Nature promotes penalties for who doesn't take precautions.
We recommend the use of tables and calculators with an estimation of energy consumption in calories. For instance an average person with 65 kg and no activity would need per day about 1.800-1.900 kilocalories, and more than 2.300 with intense exercise.
Some references are: 100 grams of fried potatoes could provide more than 300 kilocalories, or raw almonds in the same quantity more than 500. Olive oil goes to about 800 kilocalories each 100 grams.
Technically a kilocalorie are 1.000 calories or 4.184 Jules. Some people use the word calories to reference kilocalories, singularly in nutrition. If a calorie is also called “small calorie” and the kilocalorie a “large calorie”, simply stating “calorie” could fit referencing kilocalories. Anyway it's a bit imprecise.