Public domain means that no copyright exists on a given work. This is a good thing because:
- you can do whatever you want with it
- you do not need to pay for it
Of course, it's also a dual-edged weapon. You can take a public domain work, make changes, and claim it as your own copyrighted work. The original does remain in the public domain though.
In many (most?) countries, it is not possible to directly dedicate your work to the public domain. Even in countries that do allow it (such as the USA), you may still be liable for damages caused by bugs in your code. Disclaiming liability requires a license, hence taking the work out of the public domain. Given this, dedicate your work carefully, or use a license such as the X11 or BSD Licenses.
As an alternative, consider the zero license from Creative Commons: http://creativecommons.org/licenses/zero/1.0/
