Work has been done whenever there is a difference between the final kinetic energy of an object and the initial kinetic energy. Kinetic energy is the energy that an object has when moving at a certain speed. Work is a result of the conservation of energy.
There are special-case relationships between work and energy, such as the movement of an object in a straight line. If a force acts on the object and brings its velocity down to zero over a certain distance, then the work done is equal to that force times the distance it travels before it stops.