Hi there,
Any organization as per the labor law should issue an employment contract or appointment letter stating the terms of employment to the employee....
Few of the organizations just go with offer letter and do not issue an appointment letter, if a person is not issued with the appointment letter, then he can obtain a releving letter from the company which would state the joining date, end date, last drawn salary and designation.
Let me know if this information has helped you, or clarify if you need more info
Regards
V