In all cases, energy comes from one store and is transferred to another store. This means that all the energy in the Universe was present at the Big Bang and will still be around at the very end of time.
When a child comes down a slide, she begins to pick up speed. Energy is shifting from a gravitational store into a kinetic store.
However, not all of the energy is transferred into the kinetic store. As some work is done pushing against the rough surface of the slide, some of the energy is transferred to the slide and raises the temperature of the slide.
Dissipation is a term that is often used to describe ways in which energy is wasted. Any energy that is not transferred to useful energy stores is said to be wasted because it is lost to the surroundings.
Electrical cables warming up are a good example of this. It is not useful to have hot wires behind a television as energy is dissipated to the surrounding air.
In a mechanical system, energy is dissipated when two surfaces rub together. Work is done against friction which causes heating of the two surfaces - so the internal (thermal) energy of the surfaces increases. Adding lubricant between the surfaces reduces this friction and so less heat is wasted, like on a conveyor belt for example.
In an electrical context new types of component can be more energy-efficient such as LED light bulbs as opposed to filament lamps, and using these cause less energy to be wasted.
Energy is usually lost by heating up the surroundings though sometimes energy is dissipated as sound waves.
The ways in which energy is dissipated depends on the system: