In case of Esterification the nucleophile(centre loving specie that attacks on the electron deficit site) attacks on the carbocation intermediate formed during the reaction. The attack leads to the formation of a tetrahedral intermediate which on rearrangement(proton shift) and loss of water molecule, leads to the final product.
the step-wise and easy illustration is given as follows.
and evidence is also given at the last for the support of given mechanism.
Evidence for the above reaction mechanism can be understood by looking at the following reaction.