Short answers, pulled from the story.
Isaac Asimov coined the name Multivac based on a mistaken belief that UNIVAC stood for Universal Automatic Computer with vacuum tubes. He assumed Univan meant a computer with a single vacuum tube while it actually stands for Universal Automatic Computer.
The short story Franchise describes Multivac as half a mile long and three stories high. Another account states the structure spans 800 meters or fills all of Washington D.C.
Multivac chooses a single most representative person from the population of the United States to determine the country's overall orientation. The computer then calculates acceptable candidates for all elected offices based on this data.
Cosmic AC deciphered the answer after the universe died and announced Let there be light. This event essentially ascended the entity to the state of the God of the Old Testament.
Isaac Asimov's depiction of Multivac has been seen as the defining conceptualization of the genre of computers for the period 1950s through the 1960s. It served as both an oracle and a nanny in AI control terms while influencing later ideas like HAL 9000.