A hydrate is different from other chemical compounds because it is always formed by combining a substance with water. The amount or chemical state of the water in a hydrate varies depending on the compound.
Borax is a hydrate whose chemical formula is Na2B4O710H2O. Epsom salt is another hydrate that has a formula of MgSO47H2O, but there are other hydrates in existence.
Efflorescent hydrates lose water and form unhydrated substances. An example of an unhydrated substance is anhydrous copper, whose formula is CuSO4. Substances that absorb water are known as hygroscopic or deliquescent substances. Hydrates can come in different states, such as gasses or liquids.