After every interview, it is nice to write a thank you note to the interviewers who took the time out of their busy day to interview you.  

I often get asked whether one should send an email or mail a handwritten card.  For medical school interviews, my answer would be to send an email.  

1. Timeliness:  Interviewers at my school are given a deadline in which to return their evaluations.  Interviewers often have to interview multiple candidates and the admissions committee would want the interviewer to fill out an eval form as soon as possible while the applicant is still fresh in their mind.  If they wait for much longer, they may mistakenly fill out an evaluation incorrectly or they may have forgotten so much of the interview that they write 1 or 2 sentences.  A couple sentences for an interview evaluation does not give the admission committee much information to judge the applicant.
   
   Typically, interviewers are given anywhere from 1 to 3 weeks.  Email the interviewers a few days after your interviews.
    
2. Legibility: For many of us, we haven't had to use cursive since elementary school.  Therefore, our handwriting may not be optimal.  A typed email is easier read than some handwritten notes.  If you know your handwriting is horrible, then go for the email.
3. No lost mail: A hand delivered card can get lost in the campus mail.  Emails have a higher probability of getting to the inbox, but then again they could also be lost in the flood of other more important emails and spam.

So if you were to send an email or a handwritten thank you card, either method is perfectly acceptable. However, I personally would send an email.