An oxidizing agent oxides a compound, but is in turn reduced itself.
A reducing agent reduces a compound, but is in turn oxidized itself.
Think of the ETC:
You have NADH Dehydrogenase:
NADH + H+ -> NAD+
In this case, the Dehydrogenase oxidizes NADH to NAD+, and the electrons are then accepted by the enzyme, so the enzyme is reduced.
And Cytochrome oxidase (actually, a reductase):
1/2 O2 + H2 -> H2O
The reductase is reducing the electron acceptor (O2), and in turn cytochrome oxidase is being oxidized.
Basic Principle:
Redox (Reduction-Oxidation)
Knowing what happens to one thing, you can easily figure what happened to the other.