The public domain is where works with expired or no copyright protection go to retire. These works are available to be used however you like. To be clear, everything on the internet is not in the public domain, but these things are:

• Works published before 1925*
• Ideas, facts, and methods
• Federal U.S. government publications (except those written by contractors)
• Works placed in the public domain by the copyright holder
• Unpublished works by creators who died 70+ years ago
• Unpublished works by unknown/anonymous creators that were created 120+ years ago

*There are exceptions to this rule. Orphan works (where the rights holder cannot be determined) are protected. There are also some works with perpetual copyright, such as Mickey Mouse.

In general, federal government documents are in the public domain and can be freely used. They also usually included some king of copyright notice or public domain statement in the document itself.

Always check within the document to make sure there's a statement that said it's free to use (look for anything about sharing or distribution).

Note: Only federal government documents are in the public domain. This does not apply to state documents. Also, some publications are the results of contracted work, and there may still be copyright attached to it.

Creative Commons also has a statement and icon for public domain materials if creators want to use it.

The public domain notice is called a "No Rights Reserved" Creative Commons statement (not a license). 

If you see this statement or icon, it means we can use the resource without worrying about copyright.