Model releases and copyright seem to be the source of more confusion than almost any other aspects of commercial photography. Though the law in both areas is quite well established (in the U.S.), all sorts of new and insidious ideas are being bandied about on the Internet, which are to the detriment of photographers (as well as morally and legally wrong).
Most of what many people read (and say) about model releases and copyright on the Internet is dubious. I am not a lawyer. So do not consider this legal advice. But I am a photographer and a teacher. As a teacher I get paid to explain things so people who do not know about something understand that better after I explain it. With that in mind, I am going to give a broad strokes explanation of copyright and model release law. Note that I am speaking in generalities. For every rule, someone can find an exception. Folks in the know may argue with the details and especially with how sweeping I may be in what I say, but at the core, what I say is current law (in the U.S.A.)
When any creator fixes their expression/interpretation of an idea, that fixed expression is copyrighted and belongs only to the creator. The creation may be a photo, a piece of music, a sculpture, etc. The ideas themselves cannot be copyrighted. The fixed, tangible expression of those ideas can be copyrighted. In current American law, copyright exists from the moment of creation. Registering work with Library of Congress offers the creator many important additional legal protections. So much so, that I register ALL of my work with Library of Congress, to be able to take advantage of those added protections.
The creator owns that copyright unless the creator of the work goes out of their way to sign away that copyright.