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.