Which compound acts as an oxidising as well as reducing agent?

What is an oxidising agent and a reducing agent? Explain with examples.

In the reaction $Ag_2O + H_2O_2 \to 2Ag + H_2O + O_2$,the $H_2O_2$ acts as

Sulphur dioxide and hydrogen peroxide can act as both oxidizing and reducing agents in their reactions,while ozone and nitric acid act only as oxidants. Why?

Which among $KI, FeSO_4, K_2MnO_4, KMnO_4$ cannot be oxidised by $O_3$?

In the reaction $C_2O_4^{2-} + MnO_4^- + H^{+} \to Mn^{2+} + CO_2 + H_2O$,the reductant is: