Hi,
When an organization gives an offer letter to a candidate, it means the organization has selected the candidate. Now, the candidate can accept or refuse the offer. The offer letter includes the position, grade, salary, and location, etc.
An appointment letter is issued by the organization when the candidate joins the organization. It includes terms of employment such as probation, salary structure, job responsibilities (sometimes), agreements (if any), leave rules, discipline rules, termination rules, and other terms and conditions, etc.
Aashit Srivastava