Shonen (少年) literally means "few years" in Japanese. The term is often used to refer to a type of Japanese manga and anime, aimed at a young male audience under 18. Subject matter in shoujo comics vary, but a lot deal with action and adventure.
However, there are 'darker' shonen mangas around, which are aimed at an older audience. This may still be referred to as shonen in the western world, but is usually called seinen in Japan.