A hydrated crystal in one that has water molecules trapped inside the crystal structure of some other compound, typically an ionic compound.
The water is chemically combined with a substance in such a way that it can be removed, as by heating, without substantially changing the chemical composition of the substance.
As a good example, copper sulfate is a commonly hydrated crystal. To show that it is hydrated, you would write CuSO4 . 5H2O. By putting a dot in between the two, it indicates that they are waters of hydration, not really part of the compound (you can remove them by heating the crystals). You can see in this case, there are 5 water molecules for each CuSO4.