If you are a visitor over 18 with a valid driver's license from your home state, you may drive in CA for as long as you like without obtaining a California Driver's License. However, if you are between 16-18 years old then you may only drive with your out-of-state license for 10 days. After 10 days, you must have a CA driver's license or a Nonresident Minor's Certificate, which is issued by the DMV office) with proof of financial responsibility.
You do not need to pass a CA Driving Exam if you already hold a valid license in another state, as long as you are prepared to surrender your out-of-state license. If you do not have an out-of-state license, you will have to take the driver's test.
Your out-of-state permit will only be accepted if you can provide proof from your driver's education course. This requires either filling out a Form DL 33 "To Secondary Schools Other than California Schools," which you can obtain from your local DMV office or by calling 1-800-777-01333, or you can have the out-of-state secondary school write a letter confirming that teh courses you have taken are equivalent to CA secondary school course. This letter must be on the out-of-state secondary school's stationary, and it must be signed by a school official.