You can find more about all of your questions here by visiting the eNotes pages on Orwell's novel, but here is a brief overview.
Setting: Oceania, a mythical land that is something of a cross between England and America. It is set in the future, 1984, which, of course, was the future *then* as the novel was composed in 1948; (Note the inversion of the years (48; 84).
The main characters are the protagonist Winston Smith, who works for the insidious Ministry of Truth. It is Winston who will try to overcome the evils of Big Brother and his minions.
Big Brother, while never seen in person, is the totalitarian leader who controls his society through intimidation. As the novel progresses, it becomes increasingly clear that Big Brother does not truly exist.
O'Brien is the torturer whom Winston befriends. Though their relationship seems unlikely, O'Brien's intelligence and secret subversiveness attracts Wiston.
Julia is the love interest, though the relationship between she and Winston is complicated, to say the least. Julia is a member of the Anti-Sex League, but secretly loves sex.
Emmanuel Goldstein is the leader of a group trying to overthrow the ruling Party.
Tom Parson is Winston's neighbor, a sort of blue-collar worker who outwardly does not question the Party or his role, but his subconciousness betrays his desire to be free. He mumbles in his sleep, "Down with the Party."
Themes include questioning the nature of freedom and whether or not free will really exists, what constitutes loyalty and what betrayal actually means, as well as appearances vs. reality.
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