Super-Pacs are groups that may spend unlimited amounts to influence elections but may not co-ordinate with their chosen candidate’s campaign. 
They are technically independent, but can run advertisements for or against any candidate or issue. In reality, however, each candidate has a super-Pac that supports them and few voters can tell the difference between ads run by a candidate’s campaign and those by the super-Pac of its supporters.
These groups can raise unlimited amounts of money from corporations, unions, trade groups and individuals, with very loose rules dictating how and which contributions must be disclosed.
Many super-Pacs, such as Karl Rove’s American Crossroads, have created sister “charity” organisations to filter money through to further muddy the origins of their contributions. 
Up to the end of 2011, the super-political action committees set up to support President Barack Obama and Democrats running for Congress had raised $4.4m. In the space of a single week, a super-Pac backing Newt Gingrich in his bid for the Republican presidential nomination raised more than double that.