As a result of the Supreme Court’s 2010 ruling in the Citizens United v. Federal Election Commission case, rules regarding corporate and union campaign spending were significantly eased. Super PAC ads are more strongly-worded and less accurate, largely because the third party groups funding them are harder to track down than something funded directly by a candidate's campaign. Though they are not limited to one party by nature, their role in the Republican race has been striking: ads run by pro-Mitt Romney Super PAC "Restore Our Future" are widely credited with reversing Newt Gingrich's lead in Iowa.
Kathleen Hall Jamieson, director of the Annenberg Public Policy Center at the University of Pennsylvania, has more on what's fact and fiction in these ads. The Annenberg Center's flackcheck.org has outlines for each Super PAC-funded ad.