An end-of-internship thank you letter is a note of appreciation that interns ending their time with a business or organization write to show appreciation for the experience. Writing such a note shows good etiquette.
According to About.com, sending a thank you letter at the end of an internship is a potential help when it comes time to seek permanent employment. Employers tend to remember those who take the time to thank them for the opportunity to work and gain learning experience. An employer who receives such a note is likely to give strong consideration to the writer if a position is open within the company or give high marks of recommendation to another employer from whom the intern is seeking a job.