Nu Shu, also written Nüshu, is a script or writing system that was developed and used exclusively by women in China's Jiangyong county in Hunan province. Unlike the Chinese logographic script, where characters represent concepts, Nu Shu is syllabic and based on pronunciation. The script was developed several hundred years ago at a time when women and girls were not given an education. As they were not taught to read and write the Chinese script, women taught each other Nu Shu. Mothers would give their daughters booklets written in the script on their marriage, or girls would give such books to their close friends or 'sworn sisters'. The language started to die out in the 20th century as girls as well as boys were educated.
A US-based Chinese musician, Tan Dun, has spent several years collecting stories and songs written in Nu Shu, and has created a symphony from them. The BBC has a short video on it (here). For more on Nu Shu, see here.