This page lists many of the differences between Pretty Little Liars books and the Freeform TV series of the same name.
Spencer is a very intelligent girl in both the books and the TV show but in the books, as A starts stalking the girls she starts doing stupid things. She plagiarizes one of Melissa's old papers and turns it in as her own. Later, while taking summer courses at a university, she starts to develop a drug addiction from a drug nicknamed Easy A, which she buys from a guy named Phineas who is later revealed to be part of the A-Team. She then goes on to frame her roommate, Kelsey, for drug possession to avoid getting herself into trouble. She also accidentally serves pot brownies at a party at Princeton University. None of these events happen in the TV series. Another major difference is in her physical appearance. In the book series, she has dirty blonde hair and green eyes, while in the TV show, she has brown hair and brown eyes.
In the show, Ali encourages Hanna to purge occasionally, but in the books, she has a more serious issue; bulimia. She is later admitted to a mental hospital, the Preserve at Addison-Stevens by her father when he finds out about her past with bulimia, which doesn't occur in the TV series. In the books, she is also in a serious relationship with Mike Montgomery, Aria's brother, while in the TV show it was a one time fling. She also has brown hair and brown eyes in the books, while having blonde hair in blue eyes in the TV series.
In the show, Aria and Ezra have a long going relationship, while in the books the two have a brief relationship, while Ezra is still her teacher. Sean Ackard finds out about Ezra and Aria's forbidden student/teacher romance, and Ezra is taken into custody. He appears once again, asking her opinion on a book he has written about their student/teacher relationship. While her major relationship in the show is with Ezra, in the books, it's with Noel. Aria also almost elopes with a guy named Hallbjorn, who she met during her stay in Iceland. A final difference is in her physical appearance. In the TV show, she is on the smaller side, with brown hair and hazel eyes, while in the books, she is described to be tall, with black hair and blue eyes.
In both the show and the books, Emily has a crush on Ali, and goes on to date Maya once she admits to herself that she likes girls. However, in the books, when Emily's parents find out about her relationship with Maya, they send her to her aunt's house in Idaho as a punishment. There, Emily has a brief fling with a girl called Trista Taylor. Soon after Emily returns to Rosewood, her and Maya break up. In the books, Maya's death never takes place, so Emily never meets nor kills Nate (aka Lyndon). Later in the book series, Emily dates Isaac, after realizing that she is a bisexual. The two hook up and Emily becomes pregnant, and she has a baby girl who she gives up for adoption. Like the other three girls, Emily's appearance in the show and books is quite different. In the show, she has tan skin and brown eyes, while in the books, she has reddish hair, green eyes, and lots of freckles.
In both the books and TV show, Mona is regarded by Ali as a loser. After Ali's disappearance, however, she and Hanna become best friends and climb to the top of the social ladder. Another similarity between both versions of Pretty Little Liars, is that Mona is the first A, but with different motives. In the books, she wants revenge for all the years the girls treated her badly, but in the show, she is angry at the girls for stealing Hanna from her. In the books, she ends up falling off a cliff to her death, while in the show, she falls off the same cliff but ends up in a mental hospital until released. She is later thought to be dead in the show in season 5, however, it is revealed that A, Charlotte DiLaurentis, captured her and took her to the Dollhouse, forcing her to dress and act like Ali.
Jenna:The major difference with Jenna in the show and the books is that she is portrayed as more of a victim, rather than the perpetrator. It is revealed in the first arc that she was molested by her step-brother, Toby, so she asked Ali/Courtney to cause the incident known later as the Jenna Thing, to be freed from Toby. In the show, it is actually Toby who is molested by Jenna, although he is still blamed for the Jenna Thing and sent to juvenile detention in Maine. She was also one of the only people who knew that there were two sisters, Alison and Courtney since she had met them both. This secret cost her her life, when Alison came back and killed her just as she killed Ian.
In the show, Jenna would make Toby have sex with her, but in the books it was the opposite, he would molest Jenna all the time and she was too scared of him to say anything. When Alison accidentally blinded Jenna she blackmailed Toby into taking the blame; if he didn't she would tell everyone what he had done to Jenna. He is sent off to juvie and comes back a changed man. He meets Emily, and the two become friends. After going to a dance together, Toby thinks Emily has found out about his big secret, raping Jenna, so he goes into the woods and commits suicide. In the show, after becoming friends with Emily, he goes on to date Spencer and ends up falling in love with her.
In the show Ian hooked up with Spencer and Ali while he was dating Melissa for the first time, but in the show they portray him as a man with anger issues who tries to kill Spencer. In the books he and Spencer never like each other after he started dating Melissa after Ali's disappearance. Spencer thought he was A so she was glad when he was held responsible for Ali's death and sent to prison, but while he's away she realizes he's not the killer. Ian starts putting all the pieces together about the twins so the real Alison kills him.