Most Political Action Committees (PACs) represent business, labor or ideological interests.
Before the Citizens United decision, PACs traditionally collected small donations (up to $5,000 per year from any one individual) and spent a small amount per candidate (up to $5,000 each per election cycle). They can also give up to $15,000 annually to any national party committee.

But now there are Super PACs.  Technically known as independent expenditure-only committees, Super PACs may raise unlimited sums of money from corporations, unions, associations and individuals, then spend unlimited sums to overtly advocate for or against political candidates. Super PACs must, however, report their donors to the Federal Election Commission on a monthly or quarterly basis — the Super PACs choice — as a traditional PAC would. Unlike traditional PACs, Super PACs are prohibited from donating money directly to political candidates.

Here’s a link to a section of the Open Secrets web site (Center for Responsive Politics), where you can find lists of some of the donors to Super PACs and political contributions by special interest groups. As you will see if you click on the Donor tab for each Super PAC, the top donors are entrepreneur Harold Simmons and his Contrain Corporation (about $12 million to Karl Rove’s “American Crossroads”) and casino owner Sheldon Adelson and his family (about $15.5 million to Newt Gingrich’s “Winning our Future”).

Note that most of the donors are individuals. Corporations appear restrained by disclosure rules from making political donations openly, which would invite shareholder lawsuits and adverse public relations. See our page on 501(c)4 “Social Welfare Organizations for information on the equivalent of Super PACs, but which don’t require public reporting of donor names.