Most states have some form of HVAC license, which is heating, ventilation, and air conditioning. Refrigeration is sometimes added to this, and you may see the acronym HVAC/R to refer to this combination of contractor specialties.
In California, you would need to get 2 separate contractor licenses for HVAC and refrigeration. The Contractors State License Board has a stand-alone classification for Refrigeration, which is C-38. The other classification is Warm-Air Heating, Ventilating, and Air-Conditioning, which is C-20. So C-20 is basically the same as an HVAC license. Both of these are considered to be specialty licenses.
Refrigeration licenses are for those who work with refrigerators, refrigerated rooms, temperature insulation, insulated refrigerated spaces, AC units, blowers, ducts, registers, and thermostats. HVAC is for heating systems and pumps for warm water, ventilating systems with blowers and plenum chambers, AC units, and related ducts, thermostats, etc. So there is some overlap with Refrigeration and Ventilating Systems.
I will assume here that you want both of these licenses. But that is certainly not required and may not even be possible, depending on your training and experience. However, if you can apply for both, you will save a little money on the second classification. The first one will run you at about $480. And the second classification is $75. These are 2016 fees, and they are subject to change.
So if you want HVAC or refrigeration but not both, then just use one original contractor license application.
If you are going for both, use one original application and one for an additional classification.
Each classification requires a combination of 4 years of education and experience. An apprenticeship would suffice, but you need at least a year of practical experience.
HVAC has its own license exam, and so does Refrigeration. But California also requires all applicants to pass the Law and Business exam.
Test study guides are located online. Click on the appropriate links on the page posted below. The content outlines provided are not really that detailed. But specific references are provided. And these are what you can use to study for the exams.