Redox reactions is basically the short form of reduction and oxidation reactions. Both of which are the opposites of each other.

There are 4 definitions of oxidation and reduction:

Oxidation
Reduction
Gain of oxygen atom
Loss of oxygen atom
Removal of hydrogen
Addition of hydrogen
Loss of electrons
Gain of electrons
Increase in oxidation state
Decrease in oxidation state

A simple way to distinguish between Oxidation and Reduction would be through:

Oxidation Is Loss (of electrons), Reduction is Gain(of electrons). (OIL RIG)

So how do we determine the oxidation numbers?


Oxidation number
Examples
Pure element
0
O2, Zn, Al all have oxidation numbers of 0
Monatomic ions
Based on charge of ion
Cl in NaCl has an oxidation number of -1
Atoms within ions with multiple atoms in it
Based on interaction with other atoms in the ion
N in HNO3 has an oxidation number of +5