Public domains literally means: owned by the public. No one owns the copyright to a public domain work. As a member of the public, you are free to use it.
In the Untied States, there are three main ways a work can become public domain:
1) Facts and events are in the public domain.
2) Really old works for which their copyright has expired.
3) Publications created by federal government employees and agencies.
Let’s go through each of these:
Facts and Events
No one owns the right to an event in history or the quadratic equation. Here is the most important sentence regarding this topic from a court ruling “the essential ingredient present in creations, but absent in facts, is originality.” This basically means facts have no originality. No personality. The moment you add originality, you have made your own copyrighted work.
Here’s an example to better demonstrate this idea. If you read or hear an account of a true event, you can use the facts, you just can’t use the other person’s way of telling those facts. If you read a book about the Titanic sinking, you can’t copy and paste the whole book and claim public domain. The author of that book created an original way to tell those facts and the work is copyrighted. You may use all the same facts to write your own original material though.
Copyright protection only lasts so long. Once your copyright expires, into the public domain you go. But newer adaptations of that work can be copyrighted.
Here’s an example. The Grimm Brothers wrote Snow White and the Seven Dwarfs. It’s now super old. They describe the character of Snow White in immense detail, but little to no detail describing the dwarves. They didn’t even name them. Disney made an animated movie based off the material. Their character of Snow White looks almost exactly how the books describes, as to remain faithful and appease the fanbase. When it came to the dwarves, Disney obviously took a lot of liberties. Gave them names, attire, and personalities. And now Disney owns the copyright to all those dwarves. That is their original spin. If you were to make a Snow White production, you could not use Sleepy, Grumpy, Doc, Bashful, Happy, Sneezy, and Dopey. Your Snow White could look very smiliar to Disney’s Snow White, since that description matches the public domain source material. But your dwarves would have to be different.
Anything published before 1923 is in the United States public domain. So think of old books, old characters, old paintings, but be sure to distinguish what is in the source material from what is in a newer adaptation.
Federal Government Creations
Any document created by a federal government employee within the course and scope of their employment is in the public domain. But not every government document was created by a federal government employee. The government does outsource jobs. So make sure your document was prepared by a real federal government employee. And this is at the federal level, the state level is really messy.
Some documents may be unavailable because they are classified or because of privacy concerns, but they are still technically in the public domain.
Any image taken by NASA is in the public domain. That’s probably the biggest resource in all this.