From Envirowiki

Public domain is a legally defined lack of copyright, which means that anyone can use it for anything, without requiring permission of the author(s). Works often enter public domain a certain number of years after the death of the author(s), though this time span depends on the laws of the country in which the works are being used. Often, this is between 50-70 years, although some countries have no, or very little copyright legislation, effectively meaning that all works are immediately public domain.