"A public domain work is a creative work that is not protected by copyright and which may be freely used by everyone." http://www.unc.edu/~unclng/public-d.htm

Items in the Public Domain include:

1. Published works that were never copyrighted

2. Works published before 1923

3. Works published before 1989 which do not contain a notice of copyright, unless that work “has been rescued” subsequently and now is copyrighted. Any work published after March 1, 1989 is protected by copyright even if no notice of copyright is present.

4. Published works with expired copyrights: Copyrights dated 75 years or more prior to the current year may or may not have expired, depending on whether the copyright owner renewed the copyright after the first term of protection

5. Government publications

See a chart explaining "When U.S. Works Pass Into the Public Domain"

So, if it's in the public domain, you don't need to get permission to use it. Keep in mind that determining whether or not something is in the public domain can be tricky.

For example, although the Mona Lisa is not protected by copyright, a photographer would own the copyright of his or her photograph of it. Also, just because something is in the public domain does not mean that you do not need to cite it or can take credit for its creation.

For a more detailed description of public domain, see the Copyright Site.