Deliquescence is the absorption of atmospheric moisture by a solid until it dissolves to form a saturated solution. If it merely forms a crystalline hydrate it is termed hygroscopic. The phenomenon depends on the relative humidity: sugar, for example, deliquesces above 85% humidity.
Efflorescence is the spontaneous loss of water from a crystalline hydrate, which crumbles on its surface to an anhydrous powder. Sodium carbonate and sulfate are common examples. Like its converse deliquescence, it depends on the relative humidity, occurring if the partial vapor pressure of water at the solid exceeds that of air.