If you are a visitor in California, over 18, and have a valid drivers license from your home state or country, you may drive in this state without getting a California drivers license as long as your home state license remains valid. If you take a job here, like I did, or become a resident, you must get a California drivers license within 10 days.
It is so weird over here! Written tests using pen and paper to mark the anwsers to 36 questions! Then on top of that you need to bring your own car to take a "behind the wheel" test!! The instructor does a full check of your car and even asks you for hand signals....Than the instructor takes place beside you and does not even have his or her own mirrors or pedals. I can go on rambling like this, but I think you get my picture!
But to end on a good note: I already had passed the "written test" and this morning I passed my "behind the wheel" test in the U.S.A. I received an interim license valid for 60 days until I receive my actual photo license in the mail. Visit http://www.dmv.ca.gov if you want to know more about this!