Annual filing of income tax returns declaring income earned, taxes paid, and deductions claimed, applicable to individuals and all forms of business entities.
Every individual and entity earning taxable income who is statutorily required (or wishes) to file returns and claim refunds/carry-forward losses.
Income Tax Department (CBDT), Income Tax Act 1961
PAN & Aadhaar; Form 16/16A; bank statements; investment/deduction proofs (80C, 80D, etc.); business financial statements (for firms/companies); TDS certificates; previous year's ITR (if available).
Typically 31 July for individuals requiring to file ITR-1&2, 31st August for indivduals requiring to file ITR-3&4 and 31 October for entities requiring audit, though dates are notified/extended by the CBDT each year.
Late fees under Section 234F and interest on unpaid tax apply, and certain losses cannot be carried forward.