Oxidation and Reduction 

The processes of oxidation and reduction (redox reaction) involve the transfer of electrons between two substances.  The substance that loses the electrons is oxidized, and the substance that gains electrons is reduced.

The driving force for a redox reaction is the difference in the electric potential of the two substances that are involved in the electron transfer.  When two substances with different electric potentials are connected, electrons  flow from the substance with the higher electric potential to the substance with the lower electric potential.

One common type of redox reaction is the oxidation of a metal with an acid.

Mg_(s) + 2HCl_(aq) ®Â  MgCl_2(aq) + H_2(g)

In this reaction, magnesium metal loses two electrons (is oxidized) and 

each hydrogen  gains one electron (is reduced).