The water of crystallisation is referred to as the number of water molecules present in one formula unit of a salt.
For example: CaSO4.2H2O is called calcium sulphate dihydrate with and it has 2 molecules of water of crystallization.
These water molecules are held together with the compound by the special bonding called hydrogen bonding. The hydrogen bonding is a type of weak force that results in the formation of dipole-dipole interaction between a hydrogen atom and an electronegative atom that is strongly bonded to another electronegative atom. The bond or bonds are mostly strong in comparison to normal dipole-dipole and dispersion forces. Crystalline salts frequently contain water in two different states of combination, which are distinguished by the terms water of constitution (or sometimes basic water, or water of hydration), and water of crystallization. The water of crystallization is much less intimately combined with the salt than the water of the constitution and is, therefore, more easily expelled.