A law clerk is a person who assists a judge in researching issues before the court and in writing opinions. The hiring of law clerks is particularly prevalant among federal judges, and state appellate judges. Most law clerks are recent law school graduates who were near the top of their class, or who graduated from the most prestigious law schools. Most judges require that applicants for law clerk positions also have experience in law review or moot court. Judges traditionally have two law clerks, who are generally hired for a one-year or two-year term.
Because of the selection criteria, many notable legal figures, professors, and judges were law clerks before achieving greatness in other areas of the law. Several current Justices on the United States Supreme Court - William Rehnquist, Ruth Bader Ginsburg, Stephen Breyer and John Paul Stevens- were law clerks, three of whom had themselves clerked for earlier Supreme Court Justices.
Judges generally seek to hire law clerks who not only have excelled academically, but who share the judge's ideological orientation. Clerks can have a great deal of influence on the judges with whom they work. The John Grisham novel, The Tenth Justice, the title character is actually a highly influential Supreme Court law clerk.
A law clerk is not to be confused with a court clerk, as the latter occupation is essentially that of a secretary for the court.
University of Pittsburgh collection of articles on law clerks