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Hydrate- an ionic substance that has a certain number of water molecules bounded to it (it is assumed that it is one moleule of the ionic compound). Bound does not mean chemically bonded (shared valence electron) but simply held near each other due to coulombic force.
Anhydrous - it is the hydrate without the water so it really is only the ionic compound.
Percent Hydrate = (mass of H20 / mass of total sample) x 100%
Example Ferric chloride hexahydrate (FeCl3 x 6H20)
Answer: Mass of sample = 55.85g Fe + 3(35.45g Cl) + 12(1.008g H) + 6(16.00g O) = 270.296g FeCl3 x 6H20
Percent Hydrate = (12(1.008g H) + 6(16.00g O)/270.296g FeCl3 x 6H20) x 100% = 39.9917 = 40.0%