The Public Domain in an umbrella term that encompasses works that, for a number of reasons, are not protected by copyright law. This means that these works belong to the public and can be used without obtaining permission from the original author.
There are a few common ways a work enters the public domain
In order to understand how a copyright expires in the United States, we will need to go over a quick history of copyright law in the U.S.
Statute of Anne, 1710
Copyright Act of 1790
Copyright Act of 1831
Copyright Act of 1976
Copyright Term Extension Act, 1998
Under current copyright law, most works published before 1923 enter the public domain at the start of their 95th calendar year of publication.
A simple equation to remember
Ex. 2022 - 94 = 1926
This means works published before January 1, 1926 are in the public domain as of January 1, 2022.
For individual works published after January 1, 1978, copyright expires 70 years after the author has died.
Ex. If an author published work in 1978 and died in 1998, their work would enter the public domain in 2068.
1998 + 70 = 2068
For anonymous works and works made for hire published after January 1, 1978, copyright expires 95 years after the author has died OR 120 years after the publication, depending on which comes sooner.
Ex. An entertainment company produces art for an original cartoon in 1988 and the creator who made to work for hire dies in 2008, their work would enter the domain 2103.
2008 + 95 = 2103
1988 + 120 = 2108
in this case, 2103 is sooner than 2108, so that is when the copyright would expire.
There is no such thing as international copyright protection. To understand what may be in the public domain for works created in countries outside of the U.S., you will need to review that country's current copyright laws.
Most countries outside of the U.S. use an author's life + 50 years or author's life + 70 years copyright protection model. This is of course a generalization and you will need to check a country's specific permissions before using work that was produced there.
Works that are easily or "freely" accessible are not always in the public domain.
For example, if you find an image available on the internet, this does not necessarily mean it's in the public domain. In order for that image to be considered public domain, it would need to fall under its country of origin's definition of public domain or the image would need to have Creative Commons license indication that it is intended for public use.