What is the relationship between Pip and Estella in Great Expectations?

One cannot answer this question without first thinking about Miss Havisham. She was jilted as a young woman and never recovered from the loss; because of her disappointment in love, she has groomed Estella, her adoptive daughter, to break hearts as a kind of revenge on the entire male race. Dickens demonstrates this idea in Chapter 29:

"Hear me, Pip! I adopted her, to be loved. I bred her and educated her, to be loved. I developed her into what she is, that she might be loved. Love her!”

Havisham wants Pip to feel an all-consuming love that is blind and desperate, which he does. As a result of how she was raised, Estella does not have the capacity to love in return. She is cold and calloused, at one point describing herself as heartless:

“You must know,” said Estella, condescending to me as a brilliant and beautiful woman might, “that I have no heart,—if that has anything to do with my memory.”

Miss Havisham has released this beautiful woman into the world to make men feel the pain she felt. Pip falls in love with her, just as Havisham wanted him to, and Estella is aloof and indifferent. She marries someone else, but she doesn't love him either. Dickens wrote two endings, one in which Pip and Estella definitively do not end up together, and a second, less-common ending, in which there is the suggestion that they might get together.
It is worth mentioning that Dickens loves to play with names and has done so here. Estella literally means 'star.' Pip reveres and adores her, and she is bright and beautiful. But she is also distant and cold, and Pip can never actually have her.

Pip falls in love with the imperious Estella. He is the supplicant in the relationship, and she is the queen bee. This is all as Miss Havisham has planned it to be.
When Pip comes into money from a mysterious benefactor, he jumps to the conclusion that it came from Miss Havisham. He believes she wants to make him a gentleman so that he can marry Estella. Of course, nothing could be further from the truth. Miss Havisham raised Estella to break Pip's heart because Miss Havisham's own heart was broken when she was left at the altar.
The beautiful Estella, who has been taught to be cold and does not know how to love, has a haughty and high-handed relationship with Pip. She is the sun, so to speak, and he is the satellite revolving around her.