Corporations can create PACs, but cannot contribute to them.
After Citizens United, they can fund Super PACs.
They can (after the SCOTUS decision) fund media information about candidates but cannot endorse for or against any candidates
In other words, corporations can put unlimited money towards echoing a candidate's talking points, as long as they don't coordinate with with candidate. However, as Stephen Colbert [I think] pointed out, Super PACs can apparently have an office next door to a candidate, have staff who work for both, but still not be coordinating with that candidate.
As far as I can tell, the banned activities are:
Give money either directly or indirectly (via a PAC) to a candidate.
Directly endorse/oppose a candidate
But, as I have pointed out above, they can use their own money to promote the same message as a candidate.