In the US, copyright laws protect authors, artists, and composers. That means other people can’t make money from something you’ve created. You can copy snippets of another person’s work without permission (it’s called “fair use”). But anything larger than a snippet requires written permission and – in many cases – payment. The usual rule of thumb is 5% or 450 words – whichever is smaller. Be aware, though, that “fair use” has never been defined in any legal statute, so it’s wise to consult an attorney if you’re unsure about copyright protection in a particular situation.
I’ve used copyrighted materials in a number of books I’ve written. I always contacted the author to obtain permission (called a “license”). Usually the author will request a payment. I’ve also received payments myself from authors and editors who wanted permission to copy something I’ve written.
Copyright laws vary from country to country, causing additional complications. If you’re self-publishing, the best advice is to limit yourself to fair use content. In a previous post I listed some resources for royalty-free images.
(Here’s a link where you can read about an unusual experience I had while seeking copyright permission from a person whose name you’ll probably recognize.)