Energy is a physical device that relates the work done ie power over time, or power times distance.
The Joule is a unit of energy that is derived from SI units. 1 J is equivalent to the work required 1N carries over one meter.
Kcal is an older unit of energy used before Joule. The unit can be found in still in some areas.
BTU is a unit of energy used primarily in England and the United States.