Winston's diary is discussed in Part One, Chapter One, of 1984, and it is useful in introducing the reader to the nature of life under the party, a totalitarian regime, in Oceania. But the diary is significant for a number of other reasons, too. Firstly, the very fact that Winston purchased the diary is significant. As he admits, he didn't really know why he had bought it ("At the time he was not conscious of wanting it for any particular purpose"), and, perhaps, the purchase represents his unconscious desire to voice his true feelings against the party, feelings that he must ordinarily suppress for fear of committing thoughtcrime.
Secondly, what Winston writes inside the diary is of particular importance. While watching his telescreen, Winston unconsciously scribbles the phrase, DOWN WITH BIG BROTHER, over and over. By doing this, Winston's worst fear has come true: he has committed thoughtcrime and risks being shot by the Thought Police. A "kind of hysteria" now sets in and this, too, is significant. For Winston to react in this way illustrates the darker side of life under the party. They rule with an iron fist and the diary is a constant reminder of this.