Got the image by using Google
The serie is about four girls (duh) named Spencer Hastings (left on the picture), Hanna Marin (blondie), Maya St. Germain (right on the picture), Aria Montgomery (frontwoman). Before the "startdate" of the serie they had a fifth friend: Alison. One night Alison disappeared, the girls grew apart and Aria moved to Europe with her family because her dad was cheating.
When Aria returns, something strange happens.. she gets texts from -A. It seems as if Alison is still around since she knows everyones secrets. The four remaining girls have to find out what's happening and who tries to hurt them so much!