The term redox is a combination of the words reduction and oxidation. It refers to all reactions that involve the transport of electrons from an electron donor to an electron acceptor. A molecule that gets reduced gains electrons, and loses electrons when it gets oxidized (see Figure 1). For a redox reaction to occur, a molecule has to be reduced by another molecule that gets oxidized.
Figure 1: Redox reactions
OIL RIG is a common mnemonic to remember which is the oxidation and which is the reduction: Oxidation Is Loss (of electrons) and Reduction Is Gain (of electrons).
In biological systems, redox reactions are critical for cellular respiration. In the cell, electrons are often transferred via electron carriers, molecules that accept electrons from one molecule and transfer them to another.