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 out 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 thereof. In the absence of Form 16 (which should ideally be issued by the previous organization), the employee fills out 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 the 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 fall under the tax bracket. As an earning citizen of the country, you must have a PAN and submit details of your income to the Income Tax Office by filing an IT return.
Regards,
Neeraj K. Singh