How does $Sn_1$ reactions differ from $Sn_2$ reactions?

Hint: The $Sn_1$ reaction and $Sn_2$ reaction are the two types of nucleophilic substitution reaction where the suffix 1 and 2 denotes that the reaction is unimolecular reaction and bimolecular reaction.

Complete step by step answer:
The nucleophilic substitution reaction is a type of reaction where one nucleophile substitutes the other nucleophile. The nucleophilic substitution reaction is divided into $Sn_1$ reaction and $Sn_2$ reaction. The $Sn_1$ reaction is known as nucleophilic substitution reaction 1 and $Sn_2$ reaction is the nucleophilic substitution reaction 2.
In nucleophilic substitution reaction, the group that takes the electron pair and removes it from the carbon is known as the leaving group and the molecule on which the substitution takes place is known as substrate.
The difference between $Sn_1$ reaction and $Sn_2$ reactions is shown below.
$Sn_1$reaction:
(1) $Sn_1$ is a unimolecular reaction.
(2) In $Sn_1$ reaction, two steps are involved
(3) In $Sn_1$ reaction, the rate of reaction depends on the concentration of substrate.
(4) The $Sn_1$ reaction follows the first order of kinetic mechanism.
(5) In $Sn_1$ reaction, the leaving group leaves and substrate forms a carbocation intermediate.
$Sn_2$ reaction:
(1) $Sn_2$ is a bimolecular reaction.
(2) The $Sn_2$ reaction is a single step reaction.
(3) The $Sn_2$ reaction depends on the concentration of both the substrate and nucleophile.
(4) The $Sn_2$ reaction follows the second order of kinetic mechanism.
(5) The $Sn_2$ reaction takes place in a single transition state.

Note:
In the nucleophilic substitution reaction, the leaving group leaves the molecule as a neutral atom or an anion. The nucleophilic substitution reaction is the same as the displacement reaction.