They are organizations that are not supposed to be overtly partisan or political. However, in practice, they usually are. One such organization is Organizing for America (really, it's Organizing for America's Downfall!).
Here are the rules for 501(c)4's.
On a related note, universities seem to be particularly exempt from rules of basic decency. They scream about "equal pay for equal work", yet ignore on of the most egregious examples of its lack on campus.
Equal pay for equal work may be a national rallying cry. Yet for some reason, academia expects that it can pay a graduate-student teaching assistant or a PhD-holding part-time instructor a fraction of what it would pay a tenured full professor for teaching the identical class. The gulf between a full professor and a part-timer — in terms of money, power, and status — far exceeds that between the WalMart manager and his greeter at the door. And at least the latter pair have far different tasks. Is such disparity liberal?
Read more at: National Review