The Federalist Papers were a series of articles published anonymously in a New York newspaper during 1787-1788 to encourage New York to ratify the U.S. Constitution. They were also collected as a book titled "The Federalist" published as 2 volumes in 1788 (see image) and reprinted in later years. These articles, 51 by Alexander Hamilton, 29 by James Madison and 5 by John Jay, are often used today in interpreting the Constitution. All three authors wrote the articles under the pen name "Publius".
The most famous article is Federalist No. 10 by James Madison, where he argues that a union of the States will better combat factions, even factions within an individual State.
The full text of the Federalist Papers are available online.