When I first heard about sending thank you notes after an interview I could not believe that you were supposed to do this. But apparently, you are. Actually it is another opportunity to put you and your skills in front of the interviewer so that they can be reminded of what an awesome applicant that you are.
Just don't do what I just did --- double, triple and quadruple check! I misspelled the name of an interviewer on my note.... UGH. If you do that, graciously apologize, showing that you caught the slip and corrected it and were sorry for it.
Handwritten notes are the preferred method, but if time is of the essence, an email is acceptable as well. The notes should be sent within 24 hours of the interview.