What is GSTR 3B? Who should file it?

Filing GSTR 3B form is mandatory for all those who have registered for the Goods and Services Tax (GST). The GSTR 3B is a simple tax return form introduced by the Central Board of Excise and Customs (CBEC) for the month of July and August. The forms - GSTR-1, GSTR-2 and GSTR-3 - for the months of July and August are to be filed in the month of September.In the interim, all GST registrants have to file GSTR-3B form.
It is must that you have a separate GSTR 3B file for each Goods and Services Tax Identification Number (GSTIN) you have. You can mention only total values for each field in this form; invoice level information is not required for this form.
An important point to note is that some portions of Part B of GSTR-3 will be automatically populated from GSTR 3B file. So, in case there is any discrepancy between the two forms you can correct GSTR-3 later and deposit the taxes payable.
The due date for GSTR 3B for August 2017 expired on September 20. However, GSTR 3B will not be applicable starting the month of September. For the month of September, you need to file regular returns in GSTR-1, GSTR-2 and GSTR-3 file format.
Who should file GSTR 3B?
GSTR 3B must be filed by everyone who has registered for GST. However, individuals such as - Input Service Distributors, Composition Dealers, Suppliers of online information and database access or retrieval services (who have to pay tax themselves as per Section 14 of the IGST Act, and Non-resident taxable person - do not have to file GSTR 3B.