What is meant by redox reaction?

Identify the oxidizing agent and reducing agent in the given reaction, by looking at the gain and loss of oxygen : 

$2{\mathrm{Mg}}_{\left(\mathrm{s}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)} \to 2{\mathrm{MgO}}_{\left(\mathrm{s}\right)}$

Write balanced half-equations for the redox reaction given below to show the electron transfer.

Bromine displaces iodine from a solution of potassium iodide.

Write the balanced half- equations and add the half-equations, to give the ionic equation for the reaction given below.

Bromine displaces iodine from a solution of potassium iodide.

Explain the term oxidation state.