Ah, the public domain, a land of milk and honey, where the mind doesn't need to be troubled by issues of copyright. In other words, these are items that are freely available for you to use and are no longer protected by copyright. That said, before you have your milk (& honey), you have to make sure the work is actually in the public domain.
Generally, items enter the public domain for three reasons:
1) Age of the item
2) Government work
3) Creator decision
Each instance is discussed in a separate box in more detail, complete with examples.
It is possible for authors to opt out of portions or all of the copyright provisions. Currently, the best known approach is the development of Creative Commons licenses. There tag line is Share, Remix, Reuse - Legally. If you come across a CC license, that doesn't mean you have carte blanche in your use of the material. Look at the following:
You may also see a copyright symbol with a line through it. Again, with this type of symbol the author is placing the item in the public domain.
Keep in mind 2 points about the creator's decision:
1. If you do not see any symbols or otherwise duly noted, assume it is under full copyright protection.
2. Make sure the author really has permission to put the item in the pubic domain. This can be particularly true if someone found something "old" (let's say the 1960s) and figured by putting it on the web they'd make it available to everyone. You would want to double check that item to make sure it is really in the public domain.
Since copyright is fixed for a limited time, most works in the public domain are there because they have aged out of copyright protection. Unfortunately, because laws have changed over time, there is not one definitive date when all material is available. For example, a work published in the United States prior to 1923 is in the public domain, but an unpublished work is in the public domain only if the author of that work died before 1940. Sounds confusing? Well, it is complex, but fortunately there are some extraordinarily useful charts available. See the links to your left from Cornell.
Examples of items in the public domain:
Generally speaking, government documents are not protected by copyright law. Thus, as long as they aren't restricted and are publicly available, you can use them without seeking permission.
However, there is an exception. Works that have been contracted out would be works for hire. The creator of those works may still have their copyright. Or, they may sign over their rights to the US government. And finally, if a government document is freely available, companies can use that document and add value to the item. Anything added by that company would be under copyright.
So, let's look at Squeaks Discovers Type!. This is likely to still be under copyright since it was commissioned by the government agency. Who holds the copyright? It may be the author James T. Cameron and illustrator Nick Crawford, or they may have signed their copyright to the US agency. But you would need to follow up on this document.