What is copyright registration?

Copyright registration is an official recognition of copyright for the basis of a legal claim.

“The purpose of copyright registration is to place on record a verifiable account of the date and content of the work in question, so that in the event of a legal claim, or case of infringement or plagiarism, the copyright owner can produce a copy of the work from an official government source.”
— Wikipedia, Copyright registration

In America, all registrations go to the United States Copyright Office, which is a part of the Library of Congress and is based in Washington, D.C. Their website is copyright.gov.

“U.S. Copyright Office is an office of public record for copyright registration and deposit of copyright material.”
— U.S. Copyright Office

You can easily register copyrights online by uploading a group of photos and paying $45. In return, you receive a certificate of registration.

Although registration is not necessary to get copyright, it is necessary for a copyright lawsuit.