A relieving letter is issued to an employee after his separation from the company. It is a confirmation that his resignation has been accepted and his handing over work is found in order. Hence, he is relieved from his duties from that company.
A 'job experience certificate' can be very useful while applying for a new job. It is always advisable to insist on getting the experience letter from your employer just after your last day at the office (with no negative remarks as far as possible).
The experience letter generally contains the information on the job details with the company viz. designation, department, tenure of job, latest salary, key responsibilities, major achievements, etc.
In general practice, most of the experience letters contain details like designation, department and tenure of job. Other details like the latest salary, key responsibilities, major achievements, etc. are not usually mentioned in the letters.