Offer Letter - Offer Letter is a letter issued by an employer to a prospective employee. This letter basically states the position, duties and responsibilities, the pay along with terms and conditions. An offer letter becomes a valid document only once it is accepted by the prospective employee. At this stage, there is no employer-employee relationship established.
Appointment Letter - Once the offer is accepted, you will be issued an appointment letter/employment contract which officially binds you in an employer-employee relationship. This is issued and executed on your date of joining.
Reference Letter - Employment Letter/Reference Letter/Experience Letter often referred to by different names based on what is covered in the letter, is a letter which is issued after the establishment of an employer-employee relationship. This is issued either on request or when you resign from your role at the organisation.
Employment Records generally refers to supporting documents like pay stubs, hike letters, appraisal/promotion letters etc.