The iron triangle model of policy-making is one that explains how interest groups come to have great influence over the policies that are made in areas that affect them.
In this model, the interest group influences the Congress (and, more specifically, Congressional committees that are relevant to them) through things like campaign contributions and voter mobilization. For example, an interest group for farmers would work hard to get friendly members of Congress elected and placed on the Agriculture Committee.
The interest group also influences the relevant government agencies (in this example, the Department of Agriculture). It does so by lobbying Congress to use its oversight powers to make the USDA do what the farmers want. It also does so by direct contact with the bureaucrats of the USDA.
In these ways, the agricultural interest groups can gain a great deal of power over the policies that will impact them. They can influence the people who make the laws (Congress) and the people who carry them out (the governmental agencies). Many people feel this allows the interest groups to "capture" the government, making it work for them instead of allowing it to work in the public interest.
The “Iron Triangle” comprises the Bureaucratic agencies, Congressional staffs and committees and Interest groups. The triangle works by linking the three groups for mutual benefit with regards to policy making. Interest groups are known to be financiers of political campaigns (they influence voters with their funds), and it is because of this that members of Congress feel indebted to them. Thus, Congress would not normally develop policies that go against the interest groups who offer financial support during campaigns. Congress will further influence the bureaucracy and its operations through budgetary decisions and controls. The Bureaucracy will in turn implement policies that work for the Congress and the interest groups sequentially. The Bureaucracy will then enjoy support from the interest groups who have the ability to influence budgetary controls at the Congress level.