The Public Domain is a collection of works that are not subject to copyright law.
When a work is in the public domain, you can freely copy, distribute, display, and perform the work as well as make derivative and commercial works.
1. Works automatically entered into the public domain upon creation that are not copyrightable or fall under the umbrella of intellectual property (Slogans, ideas, facts, and works of federal government employees created in the scope of their employment are all examples.)
2. Works clearly donated to the public domain (Creative Commons licensed works, for example)
3. Works that have entered the public domain because the copyright on them has expired. See chart below for further explanation.
Source: Jappalang is licensed CC BY-SA 3.0