The term “public domain” refers to creative materials that are not protected by intellectual property laws such as copyright, trademark, or patent laws.
The following is a simplified flow chart to help you understand how to determine what is in the public domain. However, it should be noted that the years listed will change every January (making most things published after 1923 in the public domain).
For more information, here is an exhaustive chart to help you determine what is considered public domain.
And here is a resource to help you find works in the public domain to use in your digital projects.