The Liars

The Pretty Little Liars (or simply the Liars) is a nickname for the six girls - Alison DiLaurentis, Spencer Hastings, Aria Montgomery, Emily Fields, Hanna Marin (original members) and Mona Vanderwaal who joined on season 3 finale A DAngerous GAme.

History
As seen in Pilot, the Liars were all very close,with Alison being the ringleader. However, often times Spencer and Alison butted heads. One of the things they were responsible for was The Jenna Thing. In which, Jenna Marshall was blinded and Toby Cavanaugh was blamed for it. After Ali's disappearance, the group dissolved and it wasn't until Aria got back from Iceland and A's reign of  terror began that the friendship was resumed.

Members

 * Alison DiLaurentis (founder)
 * Spencer Hastings
 * Aria Montgomery
 * Emily Fields
 * Hanna Marin
 * Mona Vanderwaal (as of Season 4)

Allies

 * Ezra Fitz
 * Toby Cavanaugh
 * Caleb Rivers
 * Paige McCullers
 * Maya St. Germain (before her death)
 * Holden Strauss
 * Jake
 * Wesley Fitzgerald
 * Wren Kingston
 * Mike Montgomery
 * Ella Montgomery
 * Ashley Marin
 * Regina Marin
 * Veronica Hastings
 * Pam Fields
 * Wayne Fields
 * A (occasionally helps The Liars)

Storylines

 * Alison DiLaurentis, the former "Queen Bee" of her clique and the most popular girl in school before her disappearance and faked death. Although she cared for her friends, she enjoyed using their deepest secrets against them to keep them in line. Charming and manipulative, Alison was skilled at finding ways to blackmail everyone, and many people hated her. She disappeared a year prior to the beginning of the series, after which a body is found said to be hers. The show's main premise revolves around the mystery of Alison's murder. In "A DAngerous GAme," Spencer, Hanna and Mona have seen Alison and now believe her to be alive.
 * Spencer Hastings, the "smart girl" of the group. She is an extreme perfectionist who always tries to please her wealthy parents who clearly show more affection towards her older sister, Melissa. Spencer had been the only person brave enough to stand up to Alison, and may have been one of the last people to see her alive. And meanwhile she falls in love with Toby Cavanaugh
 * Aria Montgomery, the "artsy girl" of the group.  who returns to Rosewood with her family after spending a year in Iceland. At a bar, she meets Ezra Fitz and kisses him, only to discover on her first day of school that he is her English teacher. They decide to secretly continue their relationship and do so for some time, until eventually making their romance public.
 * Emily Fields, the "jock girl" of the group. Emily was closest to Alison before her disappearance and was also in love with her. A closeted lesbian, Emily was later outed by "A". She also falls in love with and dates Maya St. Germain. During this time, she also befriends Toby, Jenna Marshall's step-brother, even though she was distrustful of him at first
 * Hanna Marin, the "new It girl" of the group who struggles with self-image issues and bulimia. A "loser" while Alison was alive, Hanna subsequently transformed herself into the most popular girl in school, along with her best friend, Mona Vanderwaal. She falls in love with Caleb Rivers, a phone hacker and relative newcomer to Rosewood. Hanna is not the brightest in the group, but always sticks up for her friends
 * Mona Vanderwaal is "the new girl" and a former unpopular girl at Rosewood who longed to be accepted into Alison's group and was continuously mocked by Alison. After Alison's disappearance, Mona befriended Hanna and they underwent an "extreme makeover" together; as a result, they both became popular students. In "A DAngerous GAme", it is revealed that Mona is now being targeted by "A". As of season 4 Mona is part of the group with Spencer, Hanna, Emily, and Aria.

Season 1
In the flashback of the Pilot, the five original Liars seem very close with their leader, Alison DiLaurentis. Together, they share a tight bond, but every girl's secret deeply troubles their friendship: Spencer has a habit of crushing on, and later kissing, her older sister Melissa's boyfriends, specifically Ian, who has a secret relationship with Alison as well. Emily is a closeted lesbian harboring an unrequited crush on Alison. Hanna is an insecure bulimic, mostly due to Alison's constant taunting. Aria's father is having an affair with one of his college students, which Alison and Aria discovers, and which Alison later blackmails him for. All of the girls secrets are connected to Alison, and she is privy to them all, claiming it was their secrets that kept them close.

They all share one huge secret: The Jenna Thing. It was an incident where Alison convinced her friends to pull a prank on peeping tom Toby Cavanaugh. Ali made a firework, and planned to drop it in the Cavanaugh garage in order to scare Toby. The other girls seem reluctant to  join, due to the flimsy plan having so much room for error, yet they all give in. As they suspected, the prank went wrong, but not as badly as they imagined. The firework ended up exploding and blinding Toby's younger stepsister, Jenna. Alison blackmailed Toby into taking the blame, earning his wrath.

The bond between the girls weakened as their secrets began to tear the group apart: Spencer and Alison constantly butted heads. Alison kept the other girls secrets over their heads and even blackmailed them, leading all four girls to harbor resentment towards her. The friendship was broken with Alison's disappearance. During a sleepover, Alison served her friends drinks and after they fell asleep, she left Spencer's barn, never to be seen again. After that night, Hanna distanced herself from the other three girls and befriended outsider Mona Vanderwaal, and Aria moved with her family to Iceland. Spencer decided to dedicate herself to her studies, and Emily refocused on swimming, crushed by Ali's disappearance.

A year after Alison's disappearance, Aria moves back to Rosewood. Though she and Emily immediately pick up where they left off, Hanna and Spencer seemed uninterested. Shortly thereafter, all four girls began to receive nasty messages from the ominous A. This elusive stalker, along with Alison's body being found and the group becoming potential suspects, the bond is unbroken. Despite a rough start, the girls repair their friendship.

Soon they become as close-knit as they were before Ali's death. As a result, Hanna slightly distances herself from Mona. Their close bond is evident when Emily, Spencer and Aria grieve after Hanna is seriously injured in a hit-and-run accident. The remaining girls seem concerned when Emily goes to Homecoming with Toby, their past enemy. They alert her that it is likely he killed Alison. They are deeply saddened and angry when Emily ends up in the hospital later that night, blaming Toby for her injuries without getting the story straight. All season, it is obvious they care dearly for each other and will stop at nothing to protect each other.

Season 2
Just as in the previous season, the girl's pact is stronger than ever. They take to the fight with A, protecting even more secrets the group share and hiding them. They are all there for each other during hard times: A manages to ruin Spencer and Aria's relationships, Hanna struggles with her rivalry with her new  malicious stepsister Kate and her relationship troubles with Caleb, Emily struggles to get back on the swim team after A's drug incident and her girlfriend Maya inexplicably disappears like Alison. They group help each other bring A down, alerting Spencer, Mona turned out to be A while she was with her. They hear the altercation between them and go to the woods to help Spencer, with Hanna seriously considering ramming Mona down with her car and the girls pushing her down the cliff just as she's about to strike Spencer. By the season's end, they all rally around Emily when it is discovered Maya has been murdered.

Season 3
Despite the girls remaining strong, serious cracks begin to form in their bond: Emily drinks herself into a blackout and leads the other girls to get setup by A, making it appear as though they dug up Alison's grave. This trusts the group back into turmoil after five months of being A free. They band together yet again to fight off the new A, strong dissent begins to occur when Hanna and Spencer begin to suspect Emily's new girlfriend Paige of murdering Maya and being the new A, due to Mona planting evidence supporting their claim. It is discovered that Paige was in love with Emily long before either of them came out, despite knowing her conservative father would oppose. She wrote a love letter to her which Alison intercepted, and blackmailed her into doing her bidding. Aria joins in on their belief and stage an intervention to help Emily realize their suspicions. Emily fiercely defends Paige against her friends, and leaves to spend time with Nate, Maya's cousin who transferred to Hollis College to be involved in the investigation into her death. Emily has a good time, until Nate reveals his true intentions: he is not the cousin of Maya, he was her lover at True North, Lyndon James. He stalked her when she turned him down, and he ended up killing her. He thought he could move on with Emily, but he thinks she teased him, like Maya did. He planned on killing a bound and gagged Paige while Emily was forced to watch, however the Liars discovered  Emily was in trouble and Caleb went into the remote cabin to attempt to save Emily. While Emily had stabbed Lyndon, Caleb held her and comforted her, only to get shot by Lyndon. A sobbing Hanna is seen outside being comforted by Spencer and Aria, strengthening the group once more.

The girl's friendship did not seem to take another major hit for the remainder of the season, staying strong and it is implied they even trust former A Mona Vanderwaal to join the group in the season finale, possibly renewing her friendship with Hanna in the process.

Season 4
Mona becomes more of a role in the group, showing the Liars A's computer and telling them specific things from previous seasons. It is unknown yet whether she is a permanent member or just a helper, telling them A related information. But since the fire at the end of season 3, she has been "fired" from the A-Team which may mean she turns to the rest of the liars for help. Meanwhile the other girls are still best friends, saving each other and discovering things together. it is unknown yet whether the Ali sighting will put a wedge in their friendship but it doesn't seem so based on the promo.

Season 5
TBA

The Books
In the books, they were nicknamed first by the local news web page in Heartless after they saw Real Alison.