In the show, the girls become close after their first "A" messages. In the books it takes a while for the girls to bond, but they decide to become close friends again in Wanted, only then to have the Jamacia incident, however soon the girls become close again. They do not communicate as much in the books.
In the books, Wren is half Korean, half British, however in the show he is shown to be white British, his surname changed from Kim to Kingston.
In the books, Maya and Jenna are close friends, we never see this in the show.
In the show Toby is seen as a good person, however in the books he is perverted and creepy. In the books he is close to Emily due to the fact that they are both swimmers, but not in the show. Toby commits suicide in the books.
In the books, The Radley isn't as important, Courtney stays there for a short amount of time then it is converted into a fancy hotel. In the show, Radley is more important as both Spencer and Mona spend a lot of time there (Aria also helps out there often). In the books, Real Alison, Tabitha, Iris, Hanna and Kelsey all spend time at The Preserve, no such place exists in the show.
In the show, Ezra Fitz is a main character, whereas in the books, he is only mentioned a number of times.
In the books, Mike and Noel are both main characters, however in the show they are not very important and do not contribute much to major storylines.
In the books, Melissa and Darren get engaged and Melissa becomes pregnant with Darren's child however in the show, Melissa and Ian get married and Melissa becomes pregnant with Ian's child.
In the show, Jenna moves to Rosewood on Halloween, while the girls were already in High School, but in the books, it is presumed that Jenna has been a resident of Rosewood from a young age, as she met both Alison and Courtney around the third grade.
In the show, Alison saw Jenna as a rival, but in the books, Jenna was described as a "hopeless dork" and, like Mona, was a common target of bullying for Alison and her Posse.
In the books, Spencer, Hanna, Aria, and Emily are actually dubbed the "Pretty Little Liars" because of their numerous encounters with Ali, and are famous for them. This never happens in the show.
In the books, the Liars have been under arrest several times because of crimes that Alison has set them up for. Ali doesn't do this in the show.
In the books, Hanna is signed on as an actress for a movie titled "Burn It Down," based on their encounters with Real Ali. In the TV series, this doesn't happen, and no such movie exists.
In the TV series, Hanna has blonde hair and blue eyes, while in the books, Hanna has dyed dark auburn hair and brown eyes.
Hanna stole a pair of sunglasses in the TV series, while in the books, she stole a bracelet and earrings.
In the TV series, Hanna's personality remains the same both pre-weight loss and post-weight loss only with her gaining for confidence. In the books, Hanna acts bitchy sometimes after losing weight.
Hanna met Kate after Alison's disappearance in the TV Series, but in the books, Hanna and Alison met Kate together.
In the TV series, Hanna and Mike only hooked up, but Hanna and Mike get married in the books.
In the show, Hanna loses her virginity to Caleb however in the books she loses it to Mike.
Hanna has a closer relationship with her mother in the TV series while in the books, Mrs. Marin left Hanna for a job opportunity in Singapore.
In the TV series, it's suggested that Hanna may have only forced herself to throw up on occasion, but in the books, it was mentioned several times that Hanna is bulimic.
In the books, Hanna was sent into the Preserve because her own father believed that she might get sick again. There she finds Iris who was real Ali's friend, although she didn't know that Courtney (her Ali) had a twin.
Hanna is more popularity-obsessed in the books, while in the show she left it behind after Mona be revealed to be A, now focusing on keep her friendship with her old friends. In the books, she is not that closer with the other girls, but stop caring that much about popularity in Twisted, when she finds out she has bigger problems with a deadly new A.
In the TV series, Hanna broke her leg after being hit by the car and saw who she thought was "A", while in the books, Hanna had slight amnesia causing her to forget who "A" is. Closer to the end of the book, Aria make a comment that make Hanna has a flashback - remembering who A is.
In the books Hanna is in a car accident with Madison (Naomi Zeigler's cousin) but this does not happen in the TV series.
In the books Mona-as-A forces Hanna to tell Naomi and Riley about her issues with bulimia.
For a brief time, Hanna forms her own friendship group with Kate, Naomi and Riley.
In the show, Kate is the only one trying to mess with Hanna, however in the books they both try to ruin each others lives.
In the books Hanna is Jewish, this does not happen in the show as Hanna is seen celebrating Christmas. However it should be noted Hanna's paternal side of the family is Jewish and Hanna is non-practing Jew.
In the books Hanna's dad ran for US Senate.
In the books Hanna and Lucas have a serious relationship, whereas in the show they are friends (similar to Spencer and Andrew). However, Lucas did have feelings for Hanna that weren't reciprocated.
In the books Hanna stalks Coleen, who is dating Mike after her breakup, she eventually finds some dirt on Coleen however she doesn't spread it, only to have A spread embarrassing photo's of Hanna following Coleen. But in the end Mike takes Hanna back. This doesn't happen in the show.
In the books Hanna meets a photographer, who helps her with modeling. However it turns out he is not a real photogropher and he blackmails Hanna due to a sleazy picture he shot of her when her dress fell down. Hanna then steals money to pay and blames it on her dad's campaign advisor. In the show, this never happened.
In the TV series, Hanna dates a boy named Caleb. No such character exists in the books, though in the very first book as the Montgomery family are driving to their old house, Mike shouts, "There's Caleb's house!" However, this is likely a coincidence.
In the show, Hanna's looks seem to have a resemblance with Alison's, however in the books, Hanna looks nothing like Alison.
In the show, Hanna has a real friendship bond with Mona in the entire story, while in the books Mona only pretended to be her friend.
In the show, Hanna was briefly engaged to Jordan Hobart after the five years time jump. In the books, there is no character with this name.
In the TV series Hanna was kidnapped by Uber A after pretended to be Charlotte's Killer. In the books, this never happened.
In the books, Emily is bisexual, while in the show, she is gay.
In the books, Emily has 3 siblings but in the show she is the only child.
In the books, she has strawberry blond hair and blue eyes but in the show she has dark hair and brown eyes.
In the books, she is Caucasian; in the show, she is of Scots/Irish, Filipino and Korean descent.
In the books Emily loses her virginity to Isaac, however in the show she loses it to Maya.
In the books, Emily is sent to Iowa by her parents after they learn about her sexuality to live with her Aunt Helene, Uncle Allen and six cousins. In the show, her parents sent her girlfriend Mayaaway to a rehabilitation camp instead of Emily.
In the book Emily dated a guy named Isaac. No such character exists in the show.
Paige McCullers does not exist in the books. Thus Emily did not date anyone named Paige in the books.
In the books, Emily gets pregnant with Isaac's baby and gives up her baby for adoption but in the show no such thing has happened yet.