What is Copyright and When Does a Work Fall Into the Public Domain

Copyright is a legal tool that provides the creator of a work the right to control how a work is used. US Copyright law is a Federal Law which protects all kinds of works of authorship. This includes books, magazines, newspapers, and other writings. but can also include music, art, photographs, films/videos, computer software, maps, choreography, and architecture.
The owner of a works Copyright has the right to control how the work is reproduced, distributed, adapted, displayed, and performed. So, if someone were to use a work that is Copyrighted the owner has the right to sue for suitable compensation for any losses.
But...the copyright of a work does not last forever.
Some works are not, and never have been, protected by Copyright. For example: the works of Shakespeare. And the works of P.T. Barnum, the circus entertainer, who donated his autobiography to the Public Domain. And when a work's copyright expires, that work then falls into the Public Domain. Which means that the work can be freely copied, distributed, adapted, or performed without asking for permission or without paying any royalty fees.
Copyright law is not that difficult but it is confusing because US Copyright law has been amended so many times.
How do you first find out if a work is Copyrighted?
First you need to ascertain if a work was published.
Here is a quick overview right up until 1978 (which is the effective date of the 1976 Copyright Act):
All works published in the US before 1923 are in the Public Domain.
Works published in the US between 1923 and 1963 may be in the Public Domain. It is estimated that 85% of all works published between those years have fallen into the Public Domain.
This sounds extraordinary but for those Copyrights to stay valid, the owners had to "renew" the Copyright in the 28th. year after publication. As you can imagine, most forgot. And, many had passed on.
So, in order to check that a work's copyright had been renewed you need to either hire a specialist company to do the search for you - or, perform the search yourself. If you want to perform the search yourself, you will need access to the Internet and, depending on when the work was published, the requisite website.