If your employer deducts tax at source or TDS the tax has to be deposited with the authorities. Similarly, if a bank deducts TDS on interest income it has to deposit the same with the IT authorities.
Now, all the TDS deducted from various sources will show-up in Form 26AS. It would also show details of refund paid, advance tax paid etc.
The Form 26AS would give you an accurate amount of interest paid, salary paid and TDS. Now, let's say that you have calculated interest received from a bank and forgot to estimate the compounded returns. Let's say for example, you have invested Rs 2,00,000 in FD @10 per cent and estimate the interest income at Rs 20,000.
The Bank on the other hand has estimated the income as Rs 21,200, because they have taken into account quarterly compounding. What this means is that the interest estimated would be different in the 26 AS and would defer from your Income Tax Return. This means you have understated your income and need to pay tax on that and for sure there would be an Income Tax query. Had you to check the form 26AS it would have given you an accurate figure which is always the case.
It's therefore important to scrutinize the Form 26AS before you file your returns.
How to View Form 26 AS?
To begin with you need to visit http://contents.tdscpc.gov.in/
Please register on the same thereafter and you can then login with the password and the ID which is generally your PAN card Number. There is also a toll free number 1800 103 0344 to answer any queries. One is not sure how responsive that is and whether all queries would be entertained.