please explain sn1 and sn2

Nucleophilic substitution reactions are of two types according to the mechanisms they follow--
SN₁ reaction – SN₁ reaction is reffered to as substitution nucleophilic unimolecular reaction.In such reaction only one molecule is involved in rate determinig step and it is written as SN₁.It is found that such type of reactions are generally carried out in polar protic solvents like H2O,alcohol,acetic acid etc.
For ex- reaction between ter-butyl bromide and OH- to form tert-butyl alcohol follows such type of reaction.
(CH3)3C--Br + OH^-- ---------->. (CH3)3C--OH + Br^--
The rate of above reaction can be expressed as--
Rate = k[(CH3)3C-Br]
Since concentration of only one of the reactant involves in rate determination i.e one molecule is involved in determining the rate of raection,so it is SN₁ reaction.
SN₂ reaction– This is reffered to as substitution nucleophilic bimolecular because two molecules participate in determining the rate of the reaction.
For ex- reaction between methyl bromide and OH- to form methanol.
CH3Br + OH^- ------------> CH2OH + Br^--
Rate of the reaction is expressed as--
Rate = k[CH3Br][OH^-]
Since rate depends on the concentrations of both the reactants i.e two molecules are involved so it follows second order kinetics.