Everything dsather said sounds right on. I just wanted to add a couple of other tidbits. I have bought two rigs out of state; one in Nevada and this one in Oregon. When I did the NV transaction they would not handle any of the CA DMV registration issues. According to the finance schmuck, NV was not set up to handle CA vehicle registrations, although I heard otherwise later. So, I had to take care of the registration myself. When I bought this truck in OR, they said they could handle the DMV stuff; however, I didn't want to finance in all that extra tax and license so, once again, I did it myself. In both cases I had to get the vehicles weighed myself and provide DMV with the certificate.
Also, as dsather mentioned about the 20 day period. The DMV will expect you to come in during that period and "post fees. " Meaning, they will figure up an estimate of tax and license fees based on the paperwork you provide them and expect you to come up with that amount, pending final figures. Now this is where you can get screwed. When I registered my previous truck, the one purchased in NV, the finance paperwork never reflected the vehicles selling price, before any trade-in losses. It only showed what they had to mark it up to to cover my loss on the trade-in. So what happened is that I ended up paying taxes and fees on over $4,000 worth of trade-in loss. The dealership in OR was kind enough to provide me with a seperate bill of sale that indicated the vehicle's selling price only, no trade-in losses, and that's what DMV went off of. That saved my anus. What happens in Oregon is that even though you live in CA, when you buy a vehicle there they will register it in OR initially. You will receive the OR registration in the mail sometime during the next couple of months. CA DMV cannot finish their paperwork until you take them the OR reg.
I don't know how other states work with respect to CA, but I do know one thing, you better be prepared to shell out alot of dough when you go to the DMV.

Also, you may have to get creative with the weighing and paperwork to avoid getting screwed. :{