A Super PAC is a political action committee set up independent of any particular campaign, but able to raise and spend unlimited amounts of money in favor of the election of a candidate, provided the SuperPAC is completely separate from the campaign and the candidate, and not coordinated with them in any way. This is a result of the Citizens United v. FEC decision in January 2010.
Numerous regulations prohibit cooperation between a SuperPAC and a campaign. For example, there is a 120-day moratorium against a Super PAC hiring former employees of a campaign, under a Federal Election Commission regulation.
In the 2016 presidential race, Scott Walker's Super PAC raised $20 million by the end of July 2015, and Jeb Bush's Super PAC raised $103 million. But Scott Walker subsequently pulled out of the race on September 21.