Copyright protects creators of “original works of authorship” that are fixed in a tangible form. Copyright gives the creator exclusive rights on how their work can be accessed and used.
Copyright is granted upon creation and then placed into a fixed format. There is no need to place a copyright mark on your work for it to be copyrighted.
Copyright is automatic when the original work is created but require it to be “fixed” in a tangible form. No copyright registration is necessary.
Works can be registered for additional copyright protection. The copyright holder will receive a certificate of registration which is public record. The certificate includes the title and author of the work, copyright owner's address, year of creation, if the work has been published, previously registered, or includes preexisting material.
Possible exceptions include if the original work was considered a work for hire. Also, the creator may not be the copyright holder as they may have sold or given those rights to another entity.
The public domain contains creative work that no longer (or never had) have exclusive copyright rights. The works can be used legally by anyone without any permission from the copyright holder.
Rights do vary by country, so check copyright laws where the work is used to ensure it is in the public domain.
In addition, when works enter the public domain depend on whether they were published or unpublished. In general:
There are other circumstances, especially with published works. See Cornell's very helpful resource “Copyright Services: Copyright Term and the Public Domain in the United States”.
Under U.S. Copyright law, a creator of an "original work" created in a "fixed tangible medium" is immediately and automatically the copyright owner of the work, and your work is protected. From the moment you create your thesis or dissertation, you own the copyright; there is no need for formally register your work.
Registering a copyright to assert ownership is not required. The benefit of registration, which can be completed online, is that if your copyright is infringed you may sue for punitive damages as well as actual damages. If you do not register you may only sue for actual damages.