Question

What are hydrates?

Answer 1

Usually the word refers to salts with water 'built into' their crystals.

Explanation:

Example : Sodium carbonate `Na_2CO_3` forms a white powder. To form larger crystals, it builds in 10 molecules of water into its crystal grid, to give `Na_2CO_3*10H_2O`. This is called the hydrate.

An important hydrate is copper sulfate., because the anhydridic form `CuSO_4` is white, while the hydrate `CuSO_4*5H_2O` is blue. It is thus a reagent for water, because it picks up the water molecules quite easily.
Another one is calcium chloride `CaCl_2`, which will dry damp spaces in your home, forming `CaCl_2*2H_2O`.

Note :
This has nothing to do with carbo hydrates. They are called this way, because the ratio of `H` and `O` in them is always `2:1`, just as in water, while no real water is present.