Shōnen is a type of media meant for boys usually between the ages of about 10 and 17. It often refers to manga. Examples include Dragon Ball, Bleach, One Piece, InuYasha, Naruto, YuYu Hakusho, and Yu-Gi-Oh!.
Shounen is usually noticed by its high amounts of fighting. It often has humorous plots with male characters. It often has themes of friendship. Fighting teams and similar groups are often shown. Attractive female characters are also common, but not always present. The art style of shounen is usually less pretty than shōjo.
Beyond shōnen manga, manga for men (age 18+) is called seinen manga. Very few seinen manga have been published outside of Japan. In Japan, many older men read shounen magazines. Because of this, in Japan, shounen manga magazines are the most popular manga magazines.