If you were hired at the campus or any recruitment drive, you must have received the Letter of Intent . Its often one page document, stating the name of the role and CTC offered, date and place of joining and few more points on the document's validity. This gets termed as Offer Letter . Once you join in , you will receive, Joining Letter, with detailed information on appointment , confidentiality and compensation details. Appointment letter is issued at the end of probation, to declare the confirmation to your services to your firm.
If possible get the acknowledgement on a letter of joining. Also preserve e-mails and other correspondences exchanged.
For others who are going to JOIN: It's prudent to collect atleast the offer letter with important T & C before joining and give a joining letter mentioning the interview attended, ref.of offer letter, date when join and one shouldn't forget to collect the acknowledgement of this from the concerned/office. Wish you all the best.