There is a difference. If a candidate is selected, offer letter is issued so that he can resign from his earlier company on the basis of this letter. In offer letter, company states that the company is offering him post and he should join the company before certain date, as also, informing him to submit the required documents at the time of joining.
After joining, company collects relieving letter, experience letter and other documents from the employee and issues an Appointment letter stating period of probation if any, and other terms of employment, cessation of employment etc. In this letter company gives full details of ctc package offered to an employee.