In general, for works created since 1977, copyright lasts for the author's life plus 70 years. The purpose of this lengthy copyright period is to secure a monopoly on a copyrighted work for the author as well as her heirs/estate.
For many older works, the point at which a work was published is the point at which the clock starts ticking on copyright duration. But what does it mean for a work to be published? Well, according to the U.S. Copyright Office, publication begins when a work is made available for public distribution. For art works, publication begins at the time a work is placed on public display.
The public domain encompasses all works that are not protected by copyrighted. Works that are purely factual, such as ingredients in recipes, or a list of phone numbers, are never protected by copyright, and so go immediately into the public domain for unrestricted use by the public. Most U.S. government works as well are in the public domain, never protected by copyright. Also works that failed to comply with formalities such as registration, up to the year 1977, are in the public domain. But the big shift to the public domain occurs when a term of copyright expires: all works whose copyright term expires pass into the public domain, never to be returned to copyright protection. Works published 95 years ago or longer enter the public domain. So each year, new works are passing in to the public domain. Once a work is in the public domain, it remains there perpetually.