Hydration is the adding of water molecules to a chemical substance. The anhydrous (without water) form and the hydrated (with water) form can be quite different. For example, anhydrous tin(IV) chloride is a liquid, while the pentahydrate (with 5 water molecules) is a solid. Anhydrous copper sulfate is white or light green, while the hydrated form is blue. Anhydrous cobalt(II) chloride is blue, while the hydrated form is red. Anhydrous copper(II) chloride is brown, while the dihydrate is green.
The number of water molecules for each atom is given by a prefix. Mono- means 1, di- means 2, tri- means 3, tetra- means 4, penta- means 5, hexa- means 6, hepta- means 7, octo- means 8, nona- means 9, deca- means 10.