Oxidation-reduction reactions (often called "redox" for short) are reactions that involve the transfer of electrons from one species to another. Oxidation states, or oxidation numbers, allow chemists to keep track of these electron transfers. In general, one element will lose electrons (oxidation), with the result that it will increase in oxidation number, and another element will gain electrons (reduction), thereby decreasing in oxidation number.
An oxidizing agent is an element or compound in a redox reaction that oxidizes another species and itself gets reduced and is therefore the electron acceptor in the reaction.
A reducing agent is an element or compound in a redox reaction that reduces another species and itself gets oxidized and is therefore the electron donor in the reaction.
As a summary, keep in mind the following: