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Self Assessment Tax
The Income Tax rules function as a balance between what you need to pay (based on your earnings) and what is credited to your account (as TDS or Advance Tax). After the end of the Financial Year, you are required to balance these two sides through the process of Filing Tax Returns. During the filing, you need to compute the exact amount of tax due. If there is a shortfall in your credits, you need to pay the balance as the Self Assessment Tax.
This is called Self Assessment Tax, to distinguish it from the tax you may need to pay through an Income Tax demand later. In Self Assessment Tax, you yourself admit that your tax credit is insufficient, and pay up the balance (usually along with an interest for delayed payment).