What is The Brotherhood in 1984?

In 1984, the Brotherhood is first mentioned in Part One, Chapter One, and is described as a secret organization which rebels against the Party. Its head is Emmanuel Goldstein, the enemy of the state of Oceania:

He was the commander of a vast, shadowy army, an underground network of conspirators dedicated to the overthrow of the state.

Because of their secretive nature, the very existence of the Brotherhood is questioned by Winston. In Part...

In 1984, the Brotherhood is first mentioned in Part One, Chapter One, and is described as a secret organization which rebels against the Party. Its head is Emmanuel Goldstein, the enemy of the state of Oceania:

He was the commander of a vast, shadowy army, an underground network of conspirators dedicated to the overthrow of the state.

Because of their secretive nature, the very existence of the Brotherhood is questioned by Winston. In Part One, Chapter Seven, for instance, he refers to it as "the legendary Brotherhood" because he does not understand how its members can assemble when the watchful eye of Big Brother seems ever present. In fact, it is only when Winston goes to O'Brien's apartment in Part Two, Chapter Eight, that he changes his mind. This is because O'Brien tells him that the Brotherhood is real and that their activities, like sabotage and blackmail, are designed to "cause demoralization" and to "weaken the power of the Party."

In addition, O'Brien informs Winston that once he has read 'the book,' he will be a full member of the Brotherhood. This book, penned by Emmanuel Goldstein, outlines the corrupt nature of the Party and provides the ideological justification for rebellion. O'Brien gives a copy of the book to Winston in the next chapter.  

Unfortunately for Winston, he has been the victim of a terrible ruse: Emmanuel Goldstein and the Brotherhood are the false creations of the Party and the book is, in fact, written by O'Brien and other Party members. For his rebellion, Winston faces imprisonment and torture in the Ministry of Truth, with no hope of The Brotherhood, or any fellow rebels, coming to save him.