What Is A Difference Between Form 16,16A And 16 AA?

Please help me to know how they are deffer from each other and if some employee Salary TDS is not being deducting than which out of these need to be given.
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Hi Preeti
Form 16 is issued for the Employees/Staff whose salaries are paid directly from the company(Payroll)
Form 16A is issued to the freelancers, Contract Employee, Consultants whose income is from the third party by the company but not in the payrolls of the company but as the service for the company......
Regards,
Ramkishore

But what is 16 AA i need to know that also,because in my office they are saying it is a salary certificate is it so???