The total absence of CopyRight protection.
When something is "in the public domain", then anyone can take it, use it, copy it in any way they want. The author has given up all their exclusive rights - either voluntarily, or because the maximum term allowed by law has lapsed (normally lasts up to 70 or 90 years after the author's death, depending on which country you are in).
FreeSoftware is not in the PublicDomain; it is covered by CopyRight and CopyLeft. Shareware used to inappropriately get called 'PublicDomain software'; it definitely is not.
If you want to put your software in the PublicDomain, have a look at the CreativeCommons Public Domain license. See also SoftwareLicensing.
It should be noted that there is not, never was and probably never will be a rap group called "Public Domain". If there was, the chances of NeilHoughton liking them would be fairly high, I suppose, but this would be in spite of the fact that they have a completely ridiculous name. (CraigBox recalls there being a pisstake rap group called Public Domain in some movie, or possibly an episode of Sesame Street.)
There is, on the other hand, some kind of dance / electronica group called "Public Domain". Although it is unlikely that you are viewing this page to find out more about them, I feel it is appropriate to point out that they are largely unknown, but seem to have released two albums. "Hard Hop Superstars" was released in 2001 (apparently to little fanfare) and was followed up by "Too Many Superstars" in 2002. It seems they are also responsible for the rave theme song to the movie "Blade", although this will probably earn them more enemies than friends. I should also point out that "Blade Trinity" is absolutely the worst movie ever made, closely followed by "Blade 2". The original "Blade", a fairly crap vampire movie based on a comic book, wasn't actually all that bad.