It is a prescribed form through which the particulars of income earned by a person in a financial year and taxes paid on such income is communicated to the Income tax department after the end of the Financial year. Different forms are prescribed for filing of returns for different persons and Nature of income earned.
You should choose a return form according to your status i.e., individual/ firm/ company, etc and nature of income as explained below:
                                    ITR1    For Individuals having Income from Salary/ Pension/ family pension & Interest
ITR2    For Individuals and HUFs not having Income from Business or Profession
ITR3    For Individuals/HUFs being partners in firms and not carrying out business or profession under any proprietorship
ITR4    For individuals & HUFs having income from a proprietary business or profession
ITR5    For firms, AOPs and BOIs
ITR6    For Companies other than companies claiming exemption under section 11
ITR7    For persons including companies required to furnish return under section 139(4A) or section 139(4B) or section 139(4C) or section 139(4D)
ITRV    Where the data of the Return of Income/Fringe Benefits in Form ITR-1, ITR-2, ITR-3, ITR-4, ITR-5, ITR-6 & ITR-7 is transmitted electronically without digital signature

The new return form numbering 1 to 8 is annexure less. Hence no documents are required to be attached.
A return is to be filed before your Assessing officer. It may even be sent by post or filed electronically. Nowadays returns are also being received at designated post offices.
He/She is an officer of the Income tax department who has been given jurisdiction over a particular geographical territory or class of persons. You can find out from the PRO or from the Departmental website as to your jurisdiction i.e., the Assessing Officer with whom you need to file your Return of Income.
Companies are compulsorily required to file their return electronically. For other persons such as partnership firms/ individuals/ HUF who is required to get its accounts Audited under Income Tax law is required to file their return electronically. For electronic filing of return you have to log on to the Departmental website and upload the information of income and taxes in the prescribed form. If you have digital signature the same can be appended and there would be no need to file a hard copy physical return. In case you do not have a digital signature you will be required to file a paper return in ITR V quoting the provisional acknowledgement number received on completion of uploading of Income tax return. 
It is not necessary to be physically present while filing your Return of Income in India. However, where it is not possible for you to obtain a digital signature or sign the Return, you can authorize any person by way of a Power of Attorney to file your return. A copy of the Power of Attorney should be enclosed with the return. 
No. On the contrary by not filing your return in spite of having taxable income, you will be laying yourself open to the penal and prosecution provisions under the Income Tax Law.
Filing of return is your constitutional duty and earns for you the dignity of consciously contributing to the development of the nation. This apart, your IT returns validate your credit worthiness before financial institutions and make it possible for you to access many financial benefits such as bank credits etc.Also, even where your income is below the taxable limit but taxes have been deducted from certain receipts of income earned by you, it is necessary to file Return of Income for claiming the refund of such taxes deducted. 
If you have sustained a loss in the financial year, which you propose to carry forward to the subsequent year for adjustment against its positive income, you must make a claim of loss by filing your return before the due date of filing of Return of Income. Failure to file Return of Income may result in loss of availability of carry forward of losses to subsequent years.
The due dates are as follows:
                                    Companies & their Directors 30th September 
Other business entities, other than companies, if their accounts    30th September
are auditable & their working partners  
In all other case   31st July

For example, for Financial Year 2010-11, the due date of filing returns of income/loss is July 31, 2011 or October 31, 2011. 
Yes. This may take the form of interest if the return is not filed before the end of the assessment year. If the return is not filed even after the end of the assessment year, penalty upto Rs. 5,000/- may also be levied.
Yes. It may be furnished at any time before the expiry of two years from the end of the financial year in which the income was earned. For example, in case of income earned during FY 2010-11, the belated return can be filed before March 31, 2013.
It is never too late to start honoring your constitutional obligations for payment of tax. The department may ask you to file return of income for earlier years if it finds that you had taxable income in those years and there is tax payable by you.
The excess tax can be claimed as refund by filing your income tax return. It will be refunded by issue of cheque or by crediting to your bank account. The Income Tax Department has been making efforts to settle refund claims within four months from the month of filing return.
Yes, provided the original return has been filed before the due date and provided the department has not completed assessment of your income and taxes. However it is expected that the mistake in the original return is of a genuine and bona fide nature.
Theoretically a return can be revised any number of times before the expiry of one year from the end of the assessment year or before assessment by the department is completed; whichever event takes place earlier. 
Yes. Since legal proceedings under the income tax act can be initiated up to six years prior to the current financial year, you must maintain such documents at least for this period.
Non-payment of tax attracts interests, penalty and prosecution. The prosecution can lead to rigorous imprisonment from 6 months to 7 years and fine.