Here in the United States, copyright is granted by The Library of Congress. This is how it works - when you write something, or draw something, that you want to not be stolen and copied by someone else, you fill out a form and mail a copy of your drawing, or book, to The Library of Congress. They grant you a legal right, for a period of time, to be the only person who can copy your work. It can be renewed, up to a point, but ultimately it falls into "public domain" and anyone has the right to copy it. This system was initially set up to fill the (literally) empty Library of Congress building in Washington D.C., but as you can imagine, it got pretty crowded after a couple of hundred years. So the process of copyright went through some major simplifications, most notably in 1977 when copyright was granted upon completion of a work. You were supposed to write a C with a circle around it, or copyright, or something, but really, do didn't have to. Once you finished your drawing, or book, it was yours, and your right only to copy.
The problem with this "modern system" is enforcement. To this day, if you want your day in court to go after someone who is illegally copying your stuff, you need to have documentation from The Library of Congress. Lawyers that I have spoken to call it your "ticket to court". Yes, proof can be obtained other ways, through expert testimonial, etc., but it is wildly expensive and most lawyers won't touch a copyright infringement case without documentation.
I own the copyright on all of the illustrations and cartoons that I have done for my career - with the exception of only one client, who paid extra for the exclusive copyrights. But since I never registered anyone of my drawings with The Library of Congress, I don't stand much chance suing anyone who is using one of my cartoons. But since I do custom cartoon illustration, it doesn't matter to me. I've seen my cartoons everywhere from The Yellow Pages to the internet, and I just consider it free advertising.