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As individual, you would most likely encounter the TDS Certificate if you have fixed deposits in a Bank. The Bank would deduct tax before paying interest, and deposit this tax to the Government. As proof of this, it would give you a TDS certificate. The tax paid by the Bank on the your behalf can then be adjusted with your own tax liability when you file returns. In order for this to work correctly, the Bank should have your PAN number, so that it deposits the money correctly in your account.
Since the deadline for tax filing for salaried persons is 31 July and for others is 30 September, make sure you get your TDS Certificate, if any, well before that. In case the tax deducted is higher than your tax liability, you can claim a refund.