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:

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
Pure element
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