Stored Energy

The first law of Thermodynamics Energy is neither created nor destroyed as it can only be transferred between different forms and objects.

There are different types of energy:

Thermal which comes from heat like volcanos. 

Kinetic energy is energy coming from movement.

Gravitational potential energy, energy comes from the position of a thing in a Gravitational field.

Elastic potential energy mainly comes from springs and rubber bands. 

Chemical energy comes from chemical bonds.

Magnetic energy comes from Magnetic fields. 

Electrostatic energy which normally happens when you walk and shock someone.

And Nuclear energy comes from splitting atoms apart.

There are many ways to transfer types of energy, by using your arm to pull an elastic band, you are turning your Mechanical energy into elastic energy, electrical energy by plugging things into sockets to power them, or by light or sound waves.

When we take a group of objects, we can name that a system, in this system, energy is transferred to each object, but in an "Open System" the objects in the system can exchange energy from the outside world.

Let's take an electric kettle and fill it with water, when you turn on the power to boil the water in the kettle, the heating element uses the electric energy and turns it into heat that then transfers to the water to boil it.

There is also something named Work Done, mainly Mechanical and Electrical, mechanical work is when force is used to move an object, and electrical work happens when electricity flows between.
For example, let's say a train is moving along the track, and the tracks ahead crash down and now there is a large gap that the train will hit if it keeps going it at speed.
What the train driver will do is hit the brakes, which causes friction between the wheels and the track, slowing the train to a stop, the reason why this works is is because the kinetic energy from the train movement to thermal energy and heat due to friction.
That is why when you are going very fast and slam the breaks, the breaks themselves will heat up, and when you slide your hand fast on a smooth surface it gets hot.