From the HBO official site:
"Shoshanna is Jessa's younger, bubblier American cousin. A virgin with Sex and the City dreams and Brooklyn nightmares, her friends tend to dismiss her as a suburban innocent, but she consistently proves to be a surprising source of wisdom."
Shoshanna is the youngest member of the group.
In the third season, Shoshanna is on her final year of studies and is enjoying a "year of freedom" where she varies between academic studies and fun.
In the third season finale, Shoshanna learns that she will not be graduating after all because she failed glaciology.