Diosdado Cabello organized the pro-Chavez paramilitary organizations called "Circulos Bolivarianos".
On April 13, 2002, he took on the duties of the presidency on a temporary basis, replacing Pedro Carmona, head of the Venezuelan Chamber of Commerce, as interim president after Hugo Chavez was forced to step down. Cabello said that "I, Diosdado Cabello, am assuming the presidency until such time as the president of the republic, Hugo Chavez Frias, appears." (English translation courtesy the BBC; I don't have the Spanish text.) A few hours later, Chavez was back in office.
His background is in engineering: he has an undergraduate degree in systems engineering and a graduate degree in engineering project management.