In the United States , a Political Action Committee, or PAC, is the name commonly given to a private group, regardless of size, organized to elect political candidates. Legally, what constitutes a "PAC" for purposes of regulation is a matter of state and federal law.
Under the Federal Election Campaign Act, an organization becomes a "political committee" by receiving contributions or making expenditures in excess of $1,000 for the purpose of influencing a federal election.When an interest group wants to make political contributions, it must create a PAC. These PACs receive and raise money from the group's constituents and make donations to political campaigns. PACs, individuals, and other political committees are generally the only entities allowed to contribute funds to candidates for federal office. Contributions from corporate or labor union treasuries are illegal, though they may sponsor a PAC and provide financial support for its activities. Overall, PACs account for less than thirty percent of total contributions in U.S. Congressional races, and considerably less in presidential races.
http://en.wikipedia.org/wiki/Political_action_committee