So in my case if I have not had my Snickers Bar energy fix I have no work capacity! Energy exist in various forms potential, kinetic, thermal, electrical, chemical, nuclear, or other various forms, it may be transferred from one form to another and is always conserved. As a simple example, a car on the top of a hill has potential energy, because gravity is acting on it. The hand brake fails, it rolls down the hill loses potential energy and gains kinetic energy (movement), it hits a wall at the bottom of the hill, the car and the wall are smashed up, the energy is transferred into other energies like heat and sound as the car and wall are buckled and damaged.
Take the Snickers bar as another example, this is effectively chemical energy, which the body can process to allow me to wake up, move a little faster, work and be less grumpy. No I am not on the pay roll of Mars, Snickers just does the trick. Hay Mars, want to sponsor this site?
A better example is the use of Petrol, or Gas (for the USA) in a car. Petrol is derived from oil which is a fossil fuel and which has considerable chemical energy, because it is a carbon-based material, which will readily burn. In the car the petrol with air is ignited by spark plugs in combustion chambers, this ignition expands the gases and moves pistons, which via various mechanical components in the car drive it forward. So chemical energy transfers to kinetic energy. A car is inefficient and the process also creates a lot of thermal energy and sound energy.
The Sun is another great example to illustrate how energy transfers from one form to another. The Sun is a great big ball of gas, mostly Hydrogen and Helium. The enormous pressure and temperature in the sun provides enough energy to allow atoms of hydrogen to merge to form Helium, this process liberates even more energy from the atoms that have merged. The sun radiates some of this this energy as light (electromagnetic radiation, or photons). Some of these travel to earth. On arrival the light (or photons) may hit a leaf and transfer the energy to the plant (via photosynthesis), or hit my basking body in the Mediterranean making me hot forcing me to jump into the sea and transfer the heat to the water. They may hit the ground warming it up creating thermal currents taking with them moisture to form clouds.
You may get the idea; most energy on our planet is ultimately derived from the Sun.