You should plan on sending a thank you note to the interviewer as soon as possible after the interview has ended. In the note, reinforce your interest in the position, highlight a positive from the interview and/or clarify something that did not go as well as planned. Keep the following in mind when writing your note:
Make every effort to keep it under one page.
It may be handwritten, typed or sent by email depending on the culture of the organization. In some cases it may be appropriate to send a short email starting with something like, “Although a more formal thank you is on its way…” and then send a longer, more formal thank you card in the mail. This way, if decisions are being made immediately you have expressed your gratitude in a timely manner, while adding the polished touch of a hard copy letter - something tactile they may choose to add to your application file.
You may write to all the people with whom you interviewed or identify the primary decision maker and write to that individual. Either way, you should thank everyone with whom you interviewed in the letter.