Well.
The divides basically are the target demographic.
Shouen manga/anime is targeted towards young (10-14~) males, and it usually action packed with a protagonist that gets angry and saves people. Bleach, Naruto, etc..
Shojo manga/anime is targeted towards young (10-14~) females, and is usually more focused on relationships and such. Sailor Moon and most Magical Girl Animes are these.
Seinen manga/anime is targeted to older (18-30) males, and is very broad. Ghost in A Shell is a seinen series.
Josei manga/anime is targeted to older (18-30) females, and is generally a more mature version of Shojo. I don't have an examples of this.
They're all on Wikipedia if you want a better, in-depth view.
They aren't generally strict genre-wise, but often have common subjects or themes.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather