How do I determine the difference between substitution and elimination reaction?

Hint: The substitution reaction and the elimination reaction competes with each other. By the name of the reaction only we can determine what happens in the reaction. Substitution means to substitute and elimination means to remove.

Complete step by step answer:
The substitution reaction is also known as a single replacement reaction. In this reaction one atom or group of one reactant is substituted by another atom or group of the other reactant.
Example of a substitution reaction is the reaction of ethanol with hydrogen iodide.
The reaction between the ethanol and hydrogen iodide is shown below.
$C{H_3}C{H_2}OH + HI \to C{H_3}C{H_2}I + {H_2}O$
In this reaction, iodide ions of hydrogen ion substitute the hydroxyl functional group of ethanol to form ethyl iodide and water.
The elimination reaction is defined as the reaction where a substrate (typically an alkyl halide) eliminates one equivalent (unit) of acid to form an alkene.
Example of elimination reaction is reaction between alkyl halide with base in presence of ethanol.
$C{H_3}C{H_2}I\xrightarrow[{Ethanol}]{{NaOH}}C{H_2} = C{H_2}$
The difference between substitution and elimination reaction is shown below.


Note: In ${S_N}1$ reaction and in ${S_N}2$reaction, the S represents the chemical substitution, N represents the nucleophilic and the number represents the kinetic order of the reaction.