The biggest general difference is that NO third party public liability insurance is included in your basic car rental rate (it is included for most other states). See Liability Insurance.
Most car rental companies, in most states, provide minimum state financial liability coverage in the base rate of the rental car.. but ONLY where required by law to do so. (Most Canadian provinces include it as well.) This provides coverage for basic third party public liability and property damage coverage. California law does not require the rental car company to include basic liability coverage.
The main thing that you need to know with regards to California rental car insurance is that IF you do not carry personal automobile insurance that will extend liability coverage to your in your rental car, you will be required to purchase coverage for third party public liability and property damage claims from the car rental company.
Note: this topic addresses information for California rental car insurance as it relates to public liability coverage only... coverage for loss or damage to the rental car itself is never included in the rate... you may purchase it from the rental company, or you may already have a certain amount of coverage through your credit card or personal automobile insurance (if you have it).
You can read more about: