THE NUMBER 1 RULE
No matter what, always cite everything you did not create!!
What is the Public Domain?
Items in the public domain mean that they are not copyrighted and you can use them as you want. Most often they are items where the copyright has expired since copyright does expire after a certain amount of time.
What is Copyright?
Copyright gives authors control over their works for a limited time. This control covers copying, publishing, distributing, derivative works, performances, and format changes.
What is protected by copyright?
ASSUME EVERYTHING! Assuming everything is copyrighted makes your life easier. However, there are some things that are not copyright protected, so see the U.S. Copyright Office's Copyright Basics to get more details.
A few notes of interest: copyright happens automatically when a qualified work is created. It is copyrighted whether or not it is officially registered with the copyright office or whether or not it has the copyright symbol (©) after it. Just because it doesn't have the © after it doesn't mean it isn't copyright protected.