Chenny started when, in season 1, Chuck tried to sexually assult Jenny at a party. Since that night, Jenny and Chuck stayed away from each other, but Jenny obviously has forgiven him since then, since she let him take her virginity without reason in the finale of season 3 when they were both angry with Blair.
Chuck and Jenny appear to be civil towards each other most of the time, developing a tolerating understanding of each other. In Season 1, Chuck almost sexually assaulted Jenny, though she eventually forgave him in Season 2. When Chuck's father dies and Lily marries Rufus Humphrey, he is invited to stay with them and he does for a period of time. Chuck and Jenny don't seem to particularly like each other much, but they aren't enemies either. In Season 3, after being turned down by Nate and Chuck thinking he has been turned down by Blair, she loses her virginity to him. Though she immediately regrets this when Blair finds out.
In Season 1 Chuck tried to sexually assault Jenny until she was saved by Dan and Serena. After that they had no more contact.
When Rufus is newly engaged with Lily after Bart's death, Jenny had forgiven Chuck about what had happened last year and he told that he wouldn't try to do that again.
Jenny loses her virginity to Chuck when Jenny has been rejected by Nate, as Chuck thinks he has been rejected by Blair. Jenny immediately regrets the affair as does Chuck when Blair finds out and breaks up with him just as he is about to propose. Jenny is then sent out of the city to her mother's by Blair, Rufus and Lily by decision for her behavior to change.