An oxidation-reduction (redox) reaction involves the transfer of electrons between two species. It's essentially another name for redox reactions.
A half-reaction is either the oxidation or reduction reaction component of a redox reaction.
The substance that gains electrons in an oxidation-reduction (redox) process and causes another substance to be oxidized.
The substance that loses electrons in an oxidation-reduction (redox) process and causes another substance to be reduced.