A thank you letter is your statement of appreciation. 
It is sent within 24 hours of a formal interview or important interaction with an employer. 

The most effective thank you conveys sincere appreciation for their time, restates your interest in the position, and emphasizes your key strengths.  If you interviewed with several people during a site visit, for example -- send one letter to your initial contact, thanking the entire group. No need to send a thank you note to everyone (unless it was a very small group).

Sample thank you letters