Sending a thank you note after an interview is an opportunity for giving a final good impression. In general, you should send a note within a day or two of the interview. Even if you don't plan to accept any offered job, showing your appreciation can be a way of networking and forming good relationships with businesses with which you might have dealings in the future.
In general, the thank you note should not be too similar to a letter of inquiry or a letter preceding a resume. Instead, it should be a brief note expressing thanks for the company’s time. You can briefly allude to a few of the relevant issues in the interview, if you hope to work for the company. The note can be a bit more personal as well, and could include a statement like, “I really enjoyed discussing how you blend single parenting with working. It resembles my own philosophy.”
Such a statement should only be made when the interview has been intimate and comfortable, and when such a statement “feels” right. Another possible tactic is to thank individual employees who might have been present at the interview. Such a note to the company could say something like, “I really appreciated Bob and Shari taking time to give me a tour of the company.” Again, a more formal interview might mean referring to Bob and Shari as Mr. Anderson and Ms. Jones.
It's usually OK to make a final statement in the thank you note about how you feel you might fit in with the company. “Your philosophy on employee empowerment is closely aligned with my own," for example. The note can also imply a sense of confidence in the interview, expressing how happy you will be to hear from the company again. This can potentially save you from waiting for days when a company does not contact interviewees unless they are hired.
If you definitely plan not to accept a job after an interview, the note can serve as thanks for the time spent and as an announcement that you are withdrawing your application for a job. By still thanking the company, in a thoughtful way, you can avoid any hard feelings by not accepting a job. Any reasons that you give for not accepting the position should be vague and general. An unstable job market might one day mean applying to the same company, so even a very brief relationship with a company should end on a positive note.
Sending a thank you note shortly after an interview shows that you are considerate and pleasant to work with. Not every interviewee will take this step, and in competitive job markets, a pleasant note of thanks can be the distinction between you and other job applicants. It is a way of making a good final impression that is likely to be appreciated by a company or individual interviewers.