what is the difference between valency and the oxidation state of an element?

Oxidation state and charge are pretty much the same thing, it is a statement on the difference between the number of protons and electrons found in each atom or ion. Valency is ALMOST the same thing. Valency is the number of electrons in the outer shell of an atom and usually tells how many electrons may interact with other atoms in forming bonds. Since it is the outer electrons that tend to be removed during oxidation, then valency is a good way of predicting oxidation states. This is not always true, however, but do not focus on the exceptions, focus rather on what these terms mean.