This is simply too much to digest right at this moment. Basically, when Judge Chris Nicholson acquitted Zuma of corruption charges on 12 September 2008, he made mention that Mbeki and the justice minister had colluded with the prosecutors in the case. It was on the basis of this political interference that he threw the case out.
Zuma and his supporters have always claimed that there was a mass conspiracy to discredit him and today they have openly accused Mbeki of using the country's law enforcement system to undermine Zuma's chances of succeeding him as president. They have called for his resignation and he has apparently given it.
The New York Times mentions that an acting president will now be added from Parliament and that as Zuma is not a member of parliament he is ineligible at this stage to serve as stand-in president (he was relieved of his duties in June 2005 following corruption charges being levied against him). He can still be voted in during the next elections.