There isn't much to add to your concise classification, really.
The word itself (少女) is used to refer to a young woman, approximately 8-17 years old. Right as you might think, Shōjo manga and anime have them as target audience. Shōjo isn't limited to any particular style or styles, it's only defined by the target demographic.
This kanji (少年) is used to refer to boys from elementary school to grade school age. Shōnen manga and anime are obviously designed for them as target audience.
Seinen manga and anime are designed for male audience too, but this subset of manga and anime is made for people of older age, usually from 18 to 30, but the real audience can be even older than this with some manga. Such manga and amine usually emphasise the story and the character's personalities instead of action.
Josei manga and anime are for women aged between ~15 and ~45. They tend to be about everyday experiences of women living in Japan.
There are also sub-types of anime/manga, which tend to be more specific, for example, mahō shōjo is a sub-type of shōjo that focuses on young girl heroes with supernatural abilities (magic girls, right...).
The answer isn't really more technical than that. It's just the target demographic. The difference between a shonen manga and a seinen manga, for example, is often just a matter of which magazine it's published in.