Form 16 is a TDS certificate. Your employer is required by the I-T Department to deduct TDS on your salary and deposit it with the government.
The Form 16 certificate contains details about the salary you have earned during the year and the TDS amount deducted.
It has two parts -- Part A with details about employer and employee name, address, PAN and TAN details and TDS deductions.
Part B includes details of salary paid, other incomes, deductions allowed, tax payable.
Form 26AS is a summary of taxes deducted on your behalf and taxes paid by you. This is provided by the Income Tax Department.
It shows details of tax deducted on your behalf by deductors, details on tax deposited by taxpayers and tax refund received in the financial year. This form can be accessed from the I-T Department's website