http://www.opensecrets.org/527s/types.asp527 Group – A tax-exempt group organized under section 527 of the Internal Revenue Code to raise money for political activities including voter mobilization efforts, issue advocacy and the like. Currently, the FEC only requires a 527 group to file regular disclosure reports if it is a political party or political action committee (PAC) that engages in either activities expressly advocating the election or defeat of a federal candidate, or in electioneering communications. Otherwise, it must file either with the government of the state in which it is located or the Internal Revenue Service. Many 527s run by special interest groups raise unlimited "soft money," which they use for voter mobilization and certain types of issue advocacy, but not for efforts that expressly advocate the election or defeat of a federal candidate or amount to electioneering communications.
Political Action Committee (PAC) – A political committee that raises and spends limited "hard" money contributions for the express purpose of electing or defeating candidates. Organizations that raise soft money for issue advocacy may also set up a PAC. Most PACs represent business, such as the Microsoft PAC; labor, such as the Teamsters PAC; or ideological interests, such as the EMILY’s List PAC or the National Rifle Association PAC. An organization’s PAC will collect money from the group’s employees or members and make contributions in the name of the PAC to candidates and political parties. Individuals contributing to a PAC may also contribute directly to candidates and political parties, even those also supported by the PAC. A PAC can give $5,000 to a candidate per election (primary, general or special) and up to $15,000 annually to a national political party. PACs may receive up to $5,000 each from individuals, other PACs and party committees per year. A PAC must register with the Federal Election Commission within 10 days of its formation, providing the name and address of the PAC, its treasurer and any affiliated organizations.