A special interest group consists of people brought together by a shared belief or desire. These groups can vary in size, but they are often quite large and usually work towards making changes that will benefit their constituency. Often this type of group tries to accomplish change by influencing political processes and is often called a lobbying or advocacy group. These groups often use money and/or endorsements to try to influence politicians to approve legislation.
People with common concerns and goals can form a special interest group. When groups are brought together by these shared interests, they often try to make changes that help them reach their agreed-upon goals. This can be on a small, local scale such as a "friends of the library" group that raises funds for new books. It can also have a much larger, national focus such as the group known as "Reading is Fundamental," which is the largest non-profit organization in the U.S. with the shared interest of promoting child literacy. Other groups exist that are based on a variety of common interests including animal protection, environmental conservation, education reform, and equal rights.
Particularly when a large special interest group is involved, one of the ways to effect change is by influencing the political process. Groups like the National Rifle Association which protects the rights of gun owners, or the Sierra Club which fights for environmental issues, often have thousands of members. These members are often able to exert a great deal of influence and generate large donations to support various organizational initiatives and programs. They will also often use this power to try to influence legislation that affects their interests or to help elect politicians who share their views.
One of the ways a special interest group can influence the political process is money. These groups usually have considerable funds at their disposal and are able to hand out large political contributions. Often the unspoken price for these funds is a politician's loyalty; when legislation comes up that affects these groups, recipients of donations are expected to remember their supporters. Due to concerns about potential abuse of power, reforms have been enacted in the U.S. that limit the size of donations that can be made to individual politicians.
Another way that a special interest group can influence politics is through individual political endorsements at election time. Endorsements can influence voters when they go to cast a ballot and also provide "free" advertising for a candidate. Politicians, in turn, are often expected to assist the group in accomplishing its goals in return for this support.