What is in Room 101, as O'Brien describes it ?

O'Brien tells Winston a couple of different things with
regard to Room 101.  The first time that Winston asks O'Brien about the room, O'Brien
says that Winston (and everyone else) already knows what is in the
room.

Later on, O'Brien clarifies his remark.  He reminds
Winston of the time that Winston asked before.  But this time he explains further.  He
tells Winston that what is in Room 101 is the "worst thing in the
world."

We are not surprised when we hear that because we
have seen what the prisoners look like when they come back from Room 101 and we have
seen what they act like when they are told they have to go there.  It must be something
pretty horrible.