The Federalist Papers are a series of 85 articles, first published serially in New York state in 1787-8. The articles were intended to explain the new Constitution to the residents of New York and pursuade them to ratify it. The articles were written by James Madison, Alexander Hamilton, and John Jay. Madison is generally credited as the father of the Consitution. Hamilton was an influential delegate at the Constitutional Convention. John Jay would become the first Chief Justice of the Supreme Court.
The full text of the Federalist Papers can be found <a href="http://memory.loc.gov/const/fed/fedpapers.html">here</a>.
The Federalist Papers serve as a primary source of interpretation of the Constitution. They also outline the philosophy and motivation of the proposed system of government.