It's a formal way of expressing gratitude, and the effort you put into the card shows that you mean it. Email is too formal; thanking them in person is okay but a thank you card is ALWAYS appropriate.
Thank you cards should be relatively short, somewhat detailed and to the point. A well written thank you card can mean the difference between an okay letter of recommendation and a great one, or between being offered a job/position or not being offered one.
An example thank you card for a rotation could be something like:
Dear ______,
Thank you so much for the time you spent teaching me about retail pharmacy. I had a great time and learned so much from you. I know that the skills I learned from you will come in handy in the future and help me to be a great pharmacist. I especially enjoyed learning __________________.
Thanks again for all your time and patience.
Sincerely,
______"
Personalize it to your experience (although I woudln't just copy what I wrote) and make it your own.