Joanna's is Rachel's boss at Bloomingdale's. She's one of Rachel's interviewers in The One Where Chandler Can't Remember Which Sister, and she thinks Rachel is very competent at her job.
In The One Where They're Going To Party, Rachel is interviewed by a three-person committee for a higher position. Joanna, who is sitting on the committee, makes derogatory comments about Rachel that jeopardize her chances of getting the job. Later, Rachel confronts Joanna about the inappropriate comments Joanna made to the committee. Rachel gets so mad at Joanna's attempt to sabotage the interview, she gets ready to quit. However, Joanna tells Rachel how much she likes her as an assistant and offers her a position an assistant buyer so as not to lose her to another department. However, Joanna dies without having submitted the promotion paperwork, thus preventing Rachel from getting the personal assistant position. Furthermore, Rachel's department gets shut down at Bloomingdale's a few weeks later (in The One With Rachel's Crush). Consequently, Rachel is shunted to personal shopping, where she works until she gets a job at Ralph Lauren.
Although not much is shown to happen between Sophie and Joanna, it's quite clear that Joanna dislikes Sophie. Her snide remarks to Sophie always border on annoyance, in spite of Sophie trying to be nice to her. When Joanna dies, Sophie does not hold it any longer, and comes to work with a huge smile on her face after hearing of Joanna's demise. Rachel asks Sophie if she knows about Joana's death, and Sophie says yes.
In Season 3, Joanna meets Chandler for the first time (in The One With The Dollhouse). She's attracted to him, and after asking Rachel to set her up, Chandler agrees to go on a date with her. However, it takes just the one date for him to realize how annoying she is to him (he mentions her messy mascara goop in the corner of her eyes), but Joanna thinks he's great. After a lot of insistence from Rachel, Chandler tells Joanna that he doesn't want to call her again.
In Season 4, Chandler hooks up again with Joanna in The One With The Cuffs. This time, he sticks with her - long enough to get to know her naughtier side when she handcuffs him to her office chair. When Sophie and Rachel walk in on him half-naked and handcuffed, he pleads Rachel to free him and let him go, but Rachel, indignant that Chandler didn't keep his promise of breaking up with her boss, regrets her decision to let him go and handcuffs him to the filing cabinet. Only after promising to transform Chandler into "this generation's Milton Berle", and he agrees to stay cuffed. He does, however, get his own back and handcuffs a half-naked Joanna to her office chair after breaking up with her.