Winston's rebellion takes place on a more "intellectual" level than does Julia's. Winston is inspired by the (imaginary) work of Immanuel Goldstein. When they are in bed in the room at Charrington's, he reads part of the text to her; she falls asleep. Julia's rebellion is more at the "gut" level. She doesn't seem to care about the ideology of revolution or Goldstein or any of that. She rebels against the control the part has over her life, and her rebellion takes the form of intimate relationships with men (although we only know of Winston, she does say that she has done this with other men).
Winston and Julia chose to think for themselves in pursuing their relationship and did not heed the orders dictated by the government. Not only did they ignore common teachings that indicated that love was wrong, but they actively pursued it while being aware of the possible consequences. The fact that Winston and Julia chose to exercise their abilities of individual thought without obeying Big Brother's directives was the ultimate act of rebellion. Freedom of thought, which was considered conducive to increased individuality, was prohibited, which meant that feeling and emotion for the sake of happiness and personal promotion were also prohibited.
Fundamentally, it represents a form of individuality, and on a more direct level, their mutual love is illegal.
Their relationship is a form of "ownlife" which is antithetical to the party's ideal of a collective. Because they become emotionally attatched to each other it becomes concievable that they will put individual desires ahead of state needs. This is at the very heart of what unorthodoxy means, and is the reason for loveless marriage and "artsem". Thus, because their relationship is built on mutual attraction, sexuality, and personal consideration they are underming the heart of Engsoc. There is, however, a difference in the form rebellion takes with Winston and Julia.
In a world where there is no "I," independent thought cannot exist. The party has attempted to eliminate all relationships. Children turn their parents in, Winston's own parents "disappeared." Colleagues do not speak to each other and only interact during the Two Minute Hate. Marriage is designed for procreation only. Originally Julia and Winston are rebelling because they are selfish. Allowing for individual pleasure cannot be tolerated by Big Brother. But as they move into the Upper Room and as a relationship starts to form, they are rebelling on a much deeper level. The argument over whether they truly love each other becomes irrelevant, because they are now focused on someone other than Big Brother and that is the ultimate rebellion.