A basic letter of resignation requires a date, your effective resignation date, and your signature. In most circumstances, it's not necessary to explain your reason for leaving. Your resignation letter should be prepared and ready to submit at the time you give verbal notification. If you are resigning due to less than desirable circumstances, it's best to keep your reasons and other unnecessary statements out of print, although you may feel compelled to defend yourself. You wouldn't want to risk any termination benefits, or hopes of obtaining a future referral, at the sake of being able to blow off some steam.
Most employers file letters of resignation in case there's a problem down the road.
Therefore, never commit in writing something you may regret later. Remember to leave on good terms with other employees and management. Have your work area and files organized. The work you do at your present job will become someone else's duties once you are gone. By leaving your work area in the condition in which you would like to find it if you were the new employee, demonstrates your concern for your work and your organizational skills.