to my mind offer letters have the advantage of doing away with lengthy formalities and serving the purpose of giving cofidence that the person has been formally selected and can join duly and clarifying to him the terms of his appointment - difference between an offer letter/appointment letter do have a techncal difference in the meaning that a offer could be withdrawn before one joins which may normally done under very special circumstances only; however including the critical terms and conditions of appointment which matter to the individual and the organisation policy may well be covered in the offer letter and could well serve as an appointment letter doing away with additional letter later; it all depends on how the organisation prefers to handle matters as their internal procedure, candidates join and work if they are satisfied with the offer and based on the briefings given to them which should be transparant and complete encouraging them to unload all their queries and building mutual trust which i consider is more important rather than overworrying about the technicalities of letters. several companies give an offer letter and stop at that, a copy of which the candidate signs and returns in acceptance mentioning the likely joining date; a joining report is signed upon joining as proof of joining on the company records if the internal procedure suggests.
The general policies/practices of the company are briefed at induction, through induction guides/service manuals etc subsequently enabling the new joinee to feel familiar with the organisation and its work culture.
A well designed offer letter could enable do away with the appointment letter covering all critical aspects keeping the candidate informed, avoiding likely surprises - this could well be a matter of opinion and practice!
regards/kshantaram