A Vehicle Registration in the US allows the vehicle to be driven on public roadways, but is not the ownership document.

Maybe you've seen an American movie or TV show where a police officer pulls a driver over and says,

"License and Registration, please."

The officer is asking for the driver's Driver's License, but what does the officer mean by Registration?

A vehicle's Registration is a document that must be kept in the vehicle at all times.  

It references the vehicle's license plates and Vehicle Identification Number, and contains additional information, such as the vehicle's registered owner.

A vehicle's Registration is not an ownership document.  It is evidence of a vehicle's current owner, but Registration cannot be used to transfer ownership to a new owner - that is done by using the vehicle's Title.