One’s loss is the other’s gain

Neither oxidation nor reduction can take place without the other. When those electrons are lost, something has to gain them. Consider, for example, the net-ionic equation for a reaction with zinc metal and an aqueous copper(II) sulfate solution:

Zn(s) + Cu²⁺ → Zn²⁺ + Cu

This overall reaction is really composed of two half-reactions:

Zn(s) → Zn²⁺ + 2e^– (oxidation half-reaction — the loss of electrons)

Cu²⁺ + 2e^– → Cu(s) (reduction half-reaction — the gain of electrons)

To help yourself remember which reaction is oxidation and which is reduction in terms of electrons, memorize the phrase “LEO goes GER” (Lose Electrons Oxidation; Gain Electrons Reduction).

Zinc loses two electrons; the copper(II) cation gains those same two electrons. Zn is being oxidized. But without Cu2+ present, nothing will happen. That copper cation is the oxidizing agent. It’s a necessary agent for the oxidation process to proceed. The oxidizing agent accepts the electrons from the chemical species that’s being oxidized.  Cu²⁺ is reduced as it gains electrons. The species that furnishes the electrons is the reducing agent. In this case, the reducing agent is zinc metal.

The oxidizing agent is the species that’s being reduced, and the reducing agent is the species that’s being oxidized. Both the oxidizing and reducing agents are on the left (reactant) side of the redox equation.