It is the certificate of deduction of tax at source by any person in accordance with Chapter XVII-B of IT Act.
Form 16 is issued on deduction of tax by the employer on behalf of the employee under section 192 of IT Act, 1961.
Form 16A is issued if the deduction of tax is under any other provision of Chapter XVII-B of IT Act.
These certificates provide details of TDS / TCS for various transactions between deductor and deductee. It is mandatory to issue these certificates to Tax Payers u/s 203 of IT Act, 1961.