Who were the Federalists?
The Federalists were supporters of the ratification of the Constitution.
From 1775 until the late 1780s, the US was ruled by a one house legislature. It began as the Second Continental Congress, and after independence was won, the structure was institutionalized by the Articles of Confederation -- the first government of the United States -- which was a very weak union and the central government had very little power.
A Constitutional Convention was called to remedy the problems with the Articles, and the end product was the document we now know as the Constitution. Some felt it was too strong, others wanted it stronger. But once written, the document had to be ratified by 2/3 of the states then in the country. The people campaigning for its ratification were called Federalists, because the Constitution created a Federal Republic form of government.