The term Shoujo is a Japanese word for girl ( usually between 7-17 years old) and also it means a genre for anime and manga, targeting them as the audience.
It is usually themed and filled with beautiful boy/girls in them, where the main girl has a harem full of beautiful guys and having lots of love triangles and betrayals.
Sometimes it’s like a full pink world filled with roses and girly things, to make the viewer feel more pleasurable.
The term and anime/manga style Shoujo started to get girls into Japanese culture in the 1990s.
The entertainment industries seeing the potential in the scene, started to produce products targeting the age group which “shoujo” was meant for in Japan.
Some people say it original debut and rise in popularity can be deemed to the anime called Sailor Moon, which has first aired in 1992.
Anime and manga (Japanese culture) has huge popularity with both man and women, so when they targeted the young girl audience, they didn’t know the anime/manga style Shoujo will become so popular around the world.
Because of its popularity in the mainland and thanks to the internet and the large number of people around the world who love Japanese culture, it started to grow outside of Japan and is constantly getting translated to other languages.