The difference between a “normal” Political Action Committee and a super PAC is in the disclosure laws and uncapped “issue” expenditures, meaning a super PAC can spend whatever it can raise on targeted issues but not donate directly to a campaign. (Colbert Gets His Super Pac with Media Implications)and:
So, to review: Super-PACs focus only on politics but must disclose their donors. The 501(c) groups must not have politics as their primary purpose but don’t have to disclose who gives them money.
But it gets even more interesting when the two groups combine powers.
Say some like-minded people form both a Super-PAC and a nonprofit 501(c)(4). Corporations and individuals could then donate as much as they want to the nonprofit, which isn’t required to publicly disclose funders. The nonprofit could then donate as much as it wanted to the Super-PAC, which lists the nonprofit’s donation but not the original contributors. (Super-PACs and Dark Money: ProPublica’s Guide to the New World of Campaign Finance)
Stephen Colbert explains it quite nicely through the following three videos. If you would like the abridged version, watch the first two minutes of the last video.