Shōnen or Shounen is a Japanese word which is used to describe manga which is aimed mostly at young males (though it may have appeal to females as well). Examples include Mobile Suit Gundam and Dragon Ball. These manga are usually characterized by high action and humorous plots which center around the adventures a male hero, and/or male ensemble characters.
They may also feature unrealistically perfect females as a fanservice. (although this is much more common in seinen, manga for adult males.) Often, characters of shonen manga will be subjected to fan yaoi speculation by female fans.
The female equivalent of this type of manga is called shoujo.