Oxidation is loss of electrons from a substance, and Reduction is gain of electrons by a substance.
These two processes cannot occur without the other. If there is a reduction there must be an oxidation reaction and vice versa. The reactions usually occur simultaneously.
For example, table salt is formed by a redox reaction. Sodium is oxidized i.e. loses an electron (and becomes positively charged) while chlorine gas is reduced i.e. gains the electron (and become negatively charged). The result is formation of sodium chloride.