Redox reactions include all chemical processes in which atoms have their oxidation number (oxidation state) changed.
This can be a simple redox process, such as the oxidation of carbon to succumb carbon dioxide, it could be the decrease of carbon by hydrogen to yield methane, or it could be the oxidation of sugar in the human body, through a series of very complex electron transfer processes.

The term redox comes from the two concepts of reduction and oxidation. It can be explained in straightforward terms:

Oxidation describes the loss of an electron by a molecule, atom or ion

Reduction describes the gain of an electron by a molecule, atom or ion

They can be remembered using the mnemonic "OIL RIG"; oxidation is loss, decrease is gain.