A stalactite is a mineral formation that hangs from the ceiling of a cave, formed as water drips from the ceiling and deposits minerals. A stalagmite is a mineral formation that rises from the floor of a cave, formed from mineral deposits in water drips from stalactites above. There are four main types of stalactites and stalagmites, which differ based on shape and mineral composition, and the key difference between stalactites and stalagmites is their location - stalactites hang down from the ceiling while stalagmites rise up from the floor.