Hi Praveena,
The purpose of Form 12B and Form 16 is to detail income and tax deducted on it, however, while the former is filled up by the employee (as self declaration) the latter is issued by the Organization.
When an employee joins an Organization in the middle of the financial year, they are required to submit details of their previous income and tax deducted theirof. In absence of Form 16 (that should ideally be issued by the previous Organization), the employee fills up Form 12B detailing previous income and tax deducted on it.
Form 16 is usually issued by the Organization at the end of the Financial Year and contains details of tax deducted at source from income chargeable under the head "Salaries".
Regarding your second query, Form 16 cannot be issued without a PAN even if the employee does not come under the Tax bracket. As an earning citizen of the country, you must have a PAN and submit details of your income with the Income Tax Office by filing an IT Return.
Regards,
Neeraj K. Singh