An explosion is a process in which energy is released in an uncontrolled manner, often creating a shock wave and a loud noise. An implosion is the opposite, it is a process in which energy is drawn in, usually through a vacuum or atmospheric pressure. In an implosion, the energy created is contained within a certain area, and does not spread outwards like an explosion.
Explosions are most often associated with the combustion of fuel, such as in a car engine or with the detonation of explosives. Implosions are most commonly seen in a vacuum or atmospheric pressure situation, such as when a boiler is depressurized or when a building is demolished. In both cases, the process involves a release of energy, but with an explosion the energy is released outward, while with an implosion the energy is contained and drawn inward.