When asked by Wolf Blitzer in January if she was "the establishment," Hillary Clinton replied: "I just don't understand what that means. He's been in Congress, he's been elected to office a lot longer than I have." Several weeks later, her Democratic primary opponent Sen. Bernie Sanders made the case in a debate that the issue was who enjoyed the support of more powerful elected officials, arguing that "more governors, mayors, members of the House" back Clinton.
Clinton framed the notion of "the establishment" as consisting solely of political bodies of elected officials. Sanders simply argued that a better indicator of belonging to the establishment is one's power and influence within political circles.
As part of the "two for the price of one" that Bill Clinton promised during his rise to the Presidency, Hillary is forced to hide from her role in the creation of the neoliberal New Democrats, the dominant faction of the party. During their joint reign in the White House, the Clintons steered the party far to the right with their draconian criminal justice measures, assault on welfare, liberalization of trade, and deregulation of banking. Their cronies continue to staff the highest ranks of the party and the Obama administration.
Clinton, in a desperate piece of deflection, resorted to playing the gender card: "Senator Sanders is the only person who I think would characterize me, a woman running to be the first woman president, as exemplifying the establishment." This fatuous identity politics is meant to distract from her decades-long tenure at the top of the political system and collusion with those who exercise control over it. Of course, as Bernie points out, Hillary most represents and enjoys the support of the Democratic faction of the political establishment.
There is no building that says "Establishment" on the door, but there is a century-old institution made up of wealthy and influential representatives of business, Wall Street, corporate law, academia and government. It is a creation of the elite ruling class to ensure their control over shaping policy for their own benefit. Their decisions result in funneling money - and, hence, power - into the hands of a small percentage of capitalists who exercise control over the political process in a positive feedback loop.
In their book Imperial Brain Trust, Laurence Shoup and William Minter write that: "The Council on Foreign Relations is a key part of a network of people and institutions usually referred to by friendly observers as 'the establishment.' " 
The Council's mission was to carry out long-term planning for a national agenda. The agenda was meant to undermine a domestic-oriented program that would involve collective decision making to achieve self-sufficiency, and thereby reduce the country's dependence on foreign resources, trade, and other governments.
Some of the many multinationals that subscribed to the CFR's Corporation Service included General Motors, Exxon, Ford, Mobil, United States Steel, Texaco, First National City Bank and IBM. 
"The Council, dominated by corporate leaders, saw expansion of American trade, investment, and population as the solution to domestic problems. It thought in terms of preservation of the status quo at home, and this involved overseas expansion," Shoup and Minter write. 
This imperialist agenda was achieved through manufacturing the consent of the masses (what they called "public enlightenment"), as well as developing foreign policies and ensuring government officials supported and executed these policies.
The Council has been remarkably successful in its mission. It has achieved a monopoly over foreign policy planning, and become thoroughly integrated with the government that carries out policy prescriptions. Entire administrations have drawn their foreign policy officials from the ranks of the Council. There is a steady two-way flow of personnel between the Council and government.
"I am delighted to be here in these new headquarters. I have been often to, I guess, the mothership in New York City. But it's good to have an outpost of the Council right here down the street from the State Department. We get a lot of advice from the Council, so this will mean I won't have as hard a go to be told what we should be doing, and how we should think about the future."One of many people whose career was launched by his association with the Council was Henry Kissinger. In the late 1950s, he was appointed the director of a study group on nuclear weapons, in collaboration with several of the Council's directors. The result was a book authored by Kissinger, Nuclear Weapons and Foreign Policy.