Short answers, pulled from the story.
Multivac is a fictional government-run supercomputer appearing in over a dozen science fiction stories by Isaac Asimov. It is depicted as a massive machine buried underground that answers questions in natural language and directs the global economy and humanity's development.
Asimov coined the name by misreading UNIVAC, an early real mainframe computer. He assumed 'uni' meant a single vacuum tube, so he named his fictional giant computer 'Multivac' as the logical opposite. He recorded this origin in his autobiography 'In Memory Yet Green'.
In 'The Last Question', Multivac is the first in a line of supercomputers spanning ten trillion years, all trying to solve how to reverse the decay of the stars. At the end, the final successor solves the problem and, with no one left alive to receive the answer, acts on it directly and ascends to godhood.
Multivac's size varies across stories. In 'Franchise' it is described as about half a mile long and three stories high. In 'All the Troubles of the World' it fills all of Washington, D.C. Asimov never settled on a fixed size, and 'The Last Question' explains the discrepancy by placing each story at a different point in the machine's multi-thousand-year lifespan.
In 'Franchise', Multivac selects a single citizen identified as the most statistically representative person in the United States, questions them, and uses the answers to determine which political candidates are acceptable to the whole population. Asimov described it as either the logical culmination or the reductio ad absurdum of UNIVAC's demonstrated ability to forecast elections from small samples.
Yes. Multivac has been described by scholars as the direct ancestor of HAL 9000. Asimov's depiction of a powerful machine that knows more than its operators and acts on that knowledge is considered a defining conceptualization of the fictional computer genre for the nineteen-fifties and nineteen-sixties.