Generally speaking, your employer is not allowed to deduct money from your paycheck. Some employers try to do this anyway, taking money for things like company uniforms and tips, but this is illegal.
However, as an employee in California, you should know that state law does allow some deductions. First and foremost, if state or federal law requires it, these deductions can be made. For example, your wages could be legally garnished for back child support payments or money could be taken out to pay your income taxes.
You can also authorize deductions in many cases. This has to be done in writing, by you alone. These can be used to cover the costs of your company-based insurance premiums, for example. There could also be medical dues or hospital dues that need to be paid. However, again, this can't be done without your permission.
Finally, you may have a wage agreement in place, or, in some situations, a collective bargaining agreement. If so, this agreement can give your employer permission to make some deductions. These could be used for pension payments or health and welfare payments. You should know about these in advance, of course, since you agree to the CBA when you take the job and you know what powers it gives your employer.
If you have seen deductions coming out of your check for any other reason, you should be aware that they may be illegal. The same is true if you simply see deductions that you don't understand and that were never discussed with you. Keep a close eye on your paychecks and look into your legal options if your employer is taking money that is rightfully yours.
Source: Department of Industrial Relations, "Deductions," accessed Oct. 15, 2016