a. Super PACs
A "PAC" is a political action committee. Super PACs are in a league of their own, in a way, because they aren't connected to any particular candidates but may engage in political activity and advertising that promotes its particular political viewpoints or issues. They also may support or oppose particular candidates, but are not formally tied to any candidates' campaigns. As such they get around the normal contribution limits otherwise imposed on donating to a particular candidate or political party.
A 2010 Supreme Court decision allowed unlimited contributions to these "independent expenditure only committees," the official designation for Super PACs. The court's decision has caused quite a bit of controversy.