Oxidation and reduction are key concepts in electrochemistry. Chemistry involving oxidation and reduction is typically referred to as Redox chemistry. Oxidation involves the loss of electrons from a species and reduction involves the gain of electrons. Because we are never creating new charges out of nothing (aside from nuclear chemistry), we always have oxidation in conjunction with reduction.
Therefore in any Redox reaction, one species is always oxidized and another is reduced. We can't have one without the other. Finally, the number of electrons involved in this process must be balanced. That is, the number of electrons lost by one species needs to be the same as the number gained by the other. You can't simply build up a beaker full of electrons.
Introduction to Electrochemistry