We are first introduced to Julia during the preparation for the two minutes hate. She was "a bold-looking girl" about 27 years old, with thick dark hair, a freckled face and she moved with a swift athletic grace. Winston hated Julia ever since he had layed eyes on her. He believed that the reason he hated her was 'because of the atmosphere of hockey-fields and cold baths and community hikes and general clean mindedness which she managed to carry about her. He disliked nearly all women, and especially the young and pretty ones.... who were the most bigoted adherents of the party, the swallowers of slogans, the amateur spies and nosers-out of unorthodoxy." (pg. 12) But Winston thought there was something more dangerous about Julia than most women.
During the two minutes hate Winston gets hallucinations about trying to mortally wound Julia, and at that time he realizes why he hates her so much. It was because she represented what he could not have; she was young, strong, and beautiful.
Later in the book Julia and Winston meet in a hallway and she falls. While Winston helps her up Julia gives him a note saying "I love you". They then secretly get together and Winston finds out that Julia has had affairs with other party members, even though this is strictly forbidden in the party. From then on Winston and Julia get together many other times always switching spots so that they would not be discovered.
Although Winston and Julia both rebel against the party they are completely different in their reasoning, inspiration, and motives. Winston rebels in hope that future generations will be free of the party and be able to live in something like the Golden ages or the times he remembers from before the party took over, or at least in a time where they are free to think what they like and are not denied the privilege of the truth. Julia, being younger has no memories of a time before the party and therefore can't imagine a time without the party in control. She rebels not for the future generations, like Winston, she rebels more for the sake of just rebelling. Julia believes that the only way of rebelling against the party is with secret acts of disobedience or at the most isolated acts of violence because she doesn't believe that anyone or anything can defeat the party.
Also, although Julia is against Big Brother, she does not seem concerned about the extent of his control. She is against the party and what it stands for but she never really says her reasoning. The only time Julia questions the party's teachings when they somehow touch upon her own life. Julia also accepts lots of what the party says simply because she didn't seem to care much of the difference between a lie and the truth.
To me it seems that Winston and Julia do not completely love each other, it seems like they more love the idea of loving one another than actually having the physical and emotional connection simply because they can relate to each other. They are also very determined to stay together, when O'Brien asked if they were willing to be separated they both refused. This might be because that when they are together they know that they are not alone. It is a human need for companionship and you can not have this in the party's society. In Winston and Julia's struggle against the party, they feel they can confide in each other their actual opinions and thoughts about the party and this would make them feel much less alone.