Shoujo (少女) literally means "little female" or "little girl" in Japanese. The term is often used to refer to a type of Japanese manga and anime, aimed at a young female audience under 18. Subject matter in shoujo mangas vary, but a lot deal with romance of some kind and many are set in a high school.
However, there are 'darker' shoujo mangas around, which are aimed at an older audience. This may still be referred to as shoujo in the western world, but is usually called josei in Japan.