Big Brother and Emmanuel Goldstein are the conceptual leaders of the opposing forces in Oceania: Big Brother is the titular head of Oceania, and Goldstein is the leader of his opponents, the Brotherhood. They are similar in that Orwell does not make clear whether they actually exist.
Using doublethink, O'Brien tells Winston Smith that Big Brother does and does not exist. Big Brother does exist as the embodiment of the Party, but he can never die. O'Brien will not tell Winston whether Goldstein and the Brotherhood exists, but it is likely that both are merely Party propaganda; the fact that O'Brien claims to have written Goldstein's book is a good indication of this.
Big Brother is aptly named for his position in Oceania — a name of trust, protection, and affection — another example of doublethink. Big Brother, or, the Party, is as unlike a benevolent big brother as Hitler or Stalin. Orwell gave Emmanuel Goldstein a traditionally Jewish name that is suggestive of the power structure in World War II. Noteworthy is that Emmanuel literally means "God."
It makes no difference in Winston's life whether these two forces exist. Winston's fate is sealed, as is the fate of the society in which he lives, regardless of their existence. Big Brother and Goldstein exist in effect, and that is the only thing that matters to Winston. Orwell intended for these figures to represent totalitarian power structures; in essence, they are both the same. O'Brien, in his incarnation as a Brotherhood leader, asks Winston and Julia if they are willing to commit atrocities against the Party, many of which are no better that the atrocities that the Party commits against its people. Political extremism, as Orwell shows, is not positive under any name.