First of all, Big Brother is not a real person. Although he frequently appears on television and his picture stares out from huge posters that shout, BIG BROTHER IS WATCHING YOU, nobody sees Big Brother in person. He is a symbol of the authority of The Party.
According to cited scholars, Orwell probably had several things in mind when he created Big Brother. He was most likely thinking predominantly of Russian leader Joseph Stalin. Even the pictures of Big Brother seem to resemble him. We can speculate even further that Big Brother stands for all dictators everywhere. Orwell may as well have been thinking about figures in certain religious faiths. Particularly the judeo-christian god figure who sees and knows everything, but never appears in person.
"For Inner Party members, Big Brother is a leader, a figurehead they can use to frighten people and justify actions. For the unthinking proles, Big Brother is a distant authority figure. For Winston, Big Brother is an inspiration. Big Brother excites and energises Winston, who hates him. He is also fascinated by Big Brother and drawn to him in some of the same ways that he is drawn to O'Brien, developing a love-hate response to both of them that leads to his downfall."