Manuel Miranda’s discusses the Federalist Society in an editorial entitled The ‘Evil Cabal’ Of Conservative Lawyers:

Three years ago Sen. Orrin Hatch (R.-Utah) stood on the floor of the Senate and said: “Mr. President, I take the opportunity today to right a wrong. Over the past two years, members of the Federalist Society have been much maligned by some of my Democrat colleagues, no doubt because they see political advantage in doing so. The Federalist Society has even been presented as an ‘evil cabal’ of conservative lawyers. Its members have been subjected to questions that remind one of the McCarthy hearings of the early 1950s. Detractors have painted a picture which is surreal, twisted and untrue.”
Here we go again.
Before I proceed, let me join the view…that the left looks pretty silly in making a big deal of Judge Roberts’s association–whatever it is–with the Federalist Society. It is more than amusing; it is sophomoric. (See the three-page talking points issued yesterday by Howard Dean’s Democratic National Committee on the Roberts-Federalist cover-up.)
Judge Roberts’s ties with the Federalist Society are not the story. If Judge Roberts is not a member, he’s not a member. But the White House should not be in the business of appearing to disassociate itself from its friends. By running to correct media reports last week that Judge Roberts was a member of the Federalist Society, the White House created an issue where none existed. It should have left it to the press or Democrats to unveil this great mystery. To add injury to insult, the move now has the appearance of having been bungled with the Washington Post’s discovery of Judge Roberts’s name on a Federalist Society list from 1997-98…
There is no Federalist Society handshake and there is no Federalist Society group think. Some years ago the Washington Monthly published an article titled “The Conservative Cabal That’s Transforming American Law.” It cited a 1999 decision by the D.C. Circuit Court of Appeals as the “network’s most far-reaching victory in recent years.” The decision overturned some of the EPA’s clean-air standards on the grounds that it was unconstitutional for Congress to delegate legislative authority to the executive branch.
Oh, really? In this case two prominent individuals closely associated with the Federalist Society were of differing opinions. C. Boyden Gray, former White House Counsel for the first President Bush and a member of the Federalist Society’s Board of Visitors, filed an amicus brief making the winning argument. Yet the case was overturned by the Supreme Court in a decision written by Justice Antonin Scalia, a frequent participant in the society’s activities.
The Washington Monthly also attacked Mr. Gray as a water carrier for the Federalist Society in advancing Microsoft’s effort against antitrust enforcement. Yet Robert Bork, who also served on the Federalist Society’s Advisory Board, was Microsoft’s chief intellectual adversary.
Rather than assist the left in creating a conservative bogeyman, here is a user-friendly defense of the Federalist Society: Again, the words are Orrin Hatch’s. The Federalist Society stands for three propositions, he said: “that government’s essential purpose is the preservation of freedom; that our Constitution embraces and requires separation of governmental powers; and that judges should interpret the law, not write it. For the vast majority of Americans, these are not controversial issues.”
As Orrin Hatch concluded in his speech three years ago: The Federalist Society is “not quite the vast right-wing conspiracy hobgoblin some [Democrats] would have the American people believe.”…

Here is the link to the Federalist Society website.
They define their purpose here:

The Federalist Society for Law and Public Policy Studies is a group of conservatives and libertarians interested in the current state of the legal order. It is founded on the principles that the state exists to preserve freedom, that the separation of governmental powers is central to our Constitution, and that it is emphatically the province and duty of the judiciary to say what the law is, not what it should be.
The Society seeks both to promote an awareness of these principles and to further their application through its activities. This entails reordering priorities within the legal system to place a premium on individual liberty, traditional values, and the rule of law. It also requires restoring the recognition of the importance of these norms among lawyers, judges, and law professors.
In working to achieve these goals, the Society has created a conservative and libertarian intellectual network that extends to all levels of the legal community.

You can read more about them here, including these points:

Q. Does the Federalist Society take positions on legal or policy issues or engage in other forms of political advocacy?
A. No. The Society is about ideas. We do not lobby for legislation, take policy positions, or sponsor or endorse nominees and candidates for public service. While overall the Society believes in limited government, its members are diverse and often hold conflicting views on a broad range of issues such as tort reform, privacy rights, and criminal justice.
Q. Who joins the Federalist Society?
A. …Membership is open to anyone who wishes to join the Society, and the organization often attracts people who share a desire for public service.
Q. Who can attend Federalist Society meetings?
A. Everyone is welcome to the programs of our 180 law school chapters, 60 metropolitan lawyers chapters, and 15 nationwide practice groups. The several hundred events sponsored each year by the Federalist Society are publicly advertised and are open to the press and the general public. A number of our events are on television or are webcast.
Q. Who are some of the Federalist Society’s participants and speakers?
A. The Society has a strong reputation for hosting speakers on all sides of the ideological spectrum. A number of the Society’s most frequent and prominent speakers – from the Left as well as the Right – attest to the fact that the Society has contributed a great deal to free speech, free debate, and the public understanding of the Constitution…

Sounds like a threatening group, doesn’t it?