Though mergers and acquisitions are always used as synonyms but they do have a major difference in them.
Mergers imply combining of two similar size companies and form a new single entity. In case of merger, one or more of the existing entities loses its identity and ceases to exist and another entity or new company clubbs assets and liabilities of the merged company in its financial statement.
Whereas, in case of acquisition, none of the companies into transactions close down. Rather, the buying company obtains controlling stake into the selling company. However, the legal entity of both companies remains the same. Buying company obtains control into the acquired company through buying controlling shares and then continues the company with its changed management.