Form 16 is your annual-salary-patri. Nothing escapes from form 16 radar.
1. What is form 16?
Form 16 is a TDS certificate your employer issues to you, certifying the taxes he deducted from your salary and deposited with the central government.
2. Why is Form 16 so important to you? Form 16 is your annual-salary-patri. It is as important as your janam-patri.
Form 16 comprehensively shows all your annual salary earnings, deductions (like HRA etc) you claimed from your salary, your tax-saving investments, your taxable salary, your tax liability and taxes deducted from your salary in a few pages.
Form 16 leaves nothing, covers every minute detail your employer submitted to the tax department in his/her/it's quarterly and annual returns.
In fact, if you've ever tried filling your income tax returns using the new income tax portal's pre-filling functionality, then the pre-filled data that you see in your return is siphoned from form 16 and few other forms.
Form 16 is used as proof of income, for loan assessment and sanctioning, attached along with visa application, etc.

3. How does Form 16 look?
Form 16 comprises two parts - Part A and Part B.
Part A displays your employer’s name, address, TAN, PAN, your name, address, PAN, assessment year, employment period and summary of TDS deposited with the government.
Part B displays your salary break-up, details of deductions claimed, total taxable income and tax deducted from the salary.
4. Is there any last date for the employer to issue form 16?
Your employer is required to issue Form 16 by 31 May of the assessment year. Besides, if you switch job in the middle of any year, then the employer issues Form 16.