Crystallization is the process of formation of solid crystals by precipitating from a solution.
Water crystallization is the water that is present in crystals. Water of crystallization is the total weight of water in a substance at a given temperature and is present in a definite ratio. When some compounds crystallize from water or moist solvents (solutions), they include water in their crystals. If those compounds are heated to remove water, then crystalline properties are lost. Proteins crystallize with lots of water in them.
Gypsum is calcium sulphate dihydrate : Ca SO4 . 2 H2 O . it is used as a fertilizer and is present in plasters. When gypsum is heated, it loses water in it & becomes CaSO4 hemihydrate. Dehydrated gypsum is known as Plaster of Paris (POP). When POP is mixed with water, it again becomes gypsum after some time.
Recrystallization isused for purifying compounds. A crystal with impurities is melt / dissolved in an appropriate solvent. Then the impurities are removed. Then crystallization is done again by precipitation method.