I think it may be best to do some reading up on this website which will give you a overview of how it works and the practises it follows.
In short NASSCOM is a Database that stores records for all IT and BPO employees in India. Your potential Employer pays a figure to a NASSCOM Certified Background Checker who will check your professional employment history. And report back on the findings.
If your verification fails then it is entirley dependant on the company. I would advise having a look at the following site where you can see that "Infosys asking about 100 employees to leave due to discrepancies in their resumes". And that a "senior professional, boasting certificates from IIT Mumbai and IIM Ahmedabad and 20 years of corporate experience, being on the verge of receiving a job offer when background investigations revealed that the certificates from IIT and IIM were fake".