A redox reaction transfers electron density between chemical species.

Oxidation is loss of electrons and reduction is gain of electrons. They must occur together because electrons lost by one species are gained by another.

For example,

$$\mathrm{Zn\rightarrow Zn^{2+}+2e^-}$$

is an oxidation half-reaction, while

$$\mathrm{Cu^{2+}+2e^-\rightarrow Cu}$$

is a reduction half-reaction. Adding them cancels electrons:

$$\mathrm{Zn+Cu^{2+}\rightarrow Zn^{2+}+Cu}.$$

An oxidizing agent accepts electrons and is itself reduced; a reducing agent donates electrons and is itself oxidized.

Electron-transfer bookkeeping can also be expressed with oxidation states. A rise in oxidation state indicates oxidation; a decrease indicates reduction. Oxidation states are formal accounting labels, not necessarily literal ionic charges.

Separating a redox reaction into half-reactions is useful because charge and atoms can be balanced independently before the halves are recombined. The same framework underlies batteries, electrolysis and electrochemical corrosion.