A mushroom cloud is a large explosion comsisting of a Fireball and a stalk-like vortex of hot air. The explosion superheats the air into a vast glob of plasma, which ascends until it reaches an area of low pressure, whereupon it begins to diffuse.
As the Fireball rises, cold air is drawn in under it, along with vast ammounts of dust and ashes. Any sufficiently powerful explosion can create a mushroom cloud; however they are, in popular culture, linked to atomic detonations. Nuclear weapons create a mushroom cloud that diffuses radioactive particles in a ring about the vortex's midsection or the bottom of the fireball.