Created on or after January 1, 1978
This is governed by statutory section 17 U.S.C. 302. According to this section, a work that is created (fixed in tangible form for the first time) on or after January 1, 1978 is ordinarily given a term enduring for the author's life, plus an additional 70 years after the author's death. In the case of "a joint work" prepared by two or more authors that was not a "work made for hire," the term lasts for 70 years after the last surviving author's death. For works made for hire, and for anonymous and pseudonymous works (unless the author's identity is revealed in Copyright Office records), the duration of copyright will be 95 years from publication or 120 years from creation, whichever is shorter. For more information on joint works and works made for hire, see the BitLaw discussion on Copyright Ownership.
Created before 1978, but not published
This is governed by statutory section 17 U.S.C. 303. Works that were created but not published or registered for copyright before January 1, 1978, have been automatically brought under the statute and are now given Federal copyright protection. The duration of copyright in these works will generally be computed in the same way as for works created on or after January 1, 1978: the life-plus-70 or 95/120-year terms will apply to them as well. The law provides that in no case will the term of copyright for works in this category expire before December 31, 2002, and for works published on or before December 31, 2002, the term of copyright will not expire before December 31, 2047.
Created and published, or registered before 1978:
This is governed by statutory section 17 U.S.C. 304. Under the law in effect before 1978, copyright was secured either on the date a work was published or on the date of registration if the work was registered in unpublished form. In either case, the copyright endured for a first term of 28 years from the date it was secured. During the last (28th) year of the first term, the copyright was eligible for a second renewal term of an additional 28 years. If no application was filed for renewal, the work would enter the public domain after the initial 28 year term.
The current copyright law has extended the renewal term from 28 to 67 years for copyrights that existed as of January 1, 1978, making these works eligible for a total term of protection of 95 years. There is no longer a need to make the renewal filing in order to extend the original 28-year copyright term to the full 95 years. However, some benefits accrue to making a renewal registration during the 28th year of the original term.
In other words, if a work was published between 1923 to 1963, the copyright owner was required to have applied for a renewal term with the Copyright office. If they did not, the copyright expired and the work entered into the public domain. If they did apply for renewal, these works will have a 95 year copyright term and hence will enter into the public domain no sooner that 2018 (95 years from 1923). If the work was published between 1964 to 1977, there is no need to file for a renewal, and these works will automatically have a 95 year term.