Energy is a fundamental concept in physics just like work. It is described as the capacity of a certain system to do work. The system may either be a small boy carrying a heavy load of books or a seed slowly turning into an oak. It can also be an ocean wave crashing gently crashing into the shore or a group of kids playing ball on the field. Every living thing in the planet that moves and grows does work and is using energy. The energy that we have comes from the food that we eat or in the case of plants through photosynthesis. There is whole lot of forms of energy. There is mechanical energy which comes from machines, potential energy which is energy at rest. There is also radiant energy which is energy coming from the sun and hydrothermal energy which is energy coming from the ocean. Kinetic energy which is energy from moving matter, electrical energy which is energy coming from moving electrons and electromagnetic energy which is energy coming from light waves and etc. Energy can take on any endless form. Just like all other physics concepts, the amount of energy is measured in certain units.
There two popular units you can use in measuring energy. One is Joules while the other is Calories.
Joules was first proved by an English physicist James Prescott Joule and was subsequently named after him. Although Mr. Prescott discovered the value of Joule during his work it was not he who first introduced the term but Dr. Mayer of Heilbronn. A joule is a unit of energy used in the System Internationale which refers to the amount of energy needed for you to perform a certain kind of work when you apply a force of 1 Newton over a distance of 1 meter.
Joule should not be interchanged with Newton-meter (N.m) since the latter is used as a measure of torque while the Joule is used to describe heat and electricity. The abbreviation for joule is J in capital since it is derived from a proper name.
Meanwhile, calorie is the amount of heat that is needed to raise the temperature of 1 gram of water from 15 to 16 degrees Celsius. Calorie is typically used to measure your energy requirements and the energy content of your food. But the calories referred to in food are usually kilocalories which is equal to 1000 calories. You cannot hear the usage of calories anymore in Physics, if so you will only seldom hear it since it's considered to be outdated in contexts.
Joule now is the primary unit generally used to measure energy. For 1 joule there are 1 0.239005736 calories while for 1 calorie there is 4.2 Joules.