Two Sides of the Best Coin Ever Written

Manga is the Japanese word for comics, so we tend to refer to comics from Japan as manga. That would only make sense though wouldn't it. Generally manga can be split into two groups, although I don't find such method appropriate. The first group is shonen manga, shonen being the Japanese word for boy, and the second being shojo, their word for girl.
Shonen manga tend to be adventure type stories with less complex characters than their shojo counterparts. A typical shonen story line might involve ninjas or pirates such as Naruto and One Piece. Other common plots generes are sports stories like Initial D and The Prince of Tennis or martial arts comics such as Rurouni Kenshin and Flame of Recca. These manga will consist of many action sequences and long explanations of past events or motivations between heros and villains prior to actual combat.
Shojo manga greatly differ from shonen in a sense that they are more emotionally connected to the characters. The plots will be more deeply integrated with the feelings of the characters in the plot and twists will affect them and how they look at the world. Typical themes for shojo manga are love and dating. Good examples of such are series like Marmalade Boy and Kare Kano. Shojo series also can lean toward the perverted side of things. Many authors make sure to pack their works with plenty of panty flashes and unproportioned females. We refer to such things as fan service. You can find these kinds of characters in shonen manga as well, they just tend to be kicking someone's ass most of the time.
For all their differences shojo and shonen manga have many similarities as well. Characters in more up beat series have a goal that they are striving toward. A common character type in both kinds is the optimistic fighting dreamer. They've had a goal most their entire lives and every day is devoted to accomplishing that goal. Sho Kazamatsuri from the shonen soccer series Whistle! and Nobara Sumiyoshi from the shojo series Crimson Hero are excellent examples of optimistic characters.
Another theme shared by shonen and shojo manga is friendship. It is main theme of many series like Yu-Gi-Oh! Where as our story open the main character, Yugi Mutoh, is wishing for friends of his own. In the shojo series Marmalade Boy the MC, Miki Koishikawa, relies heavily on her best friend Meiko Suzuki. The two friends share a journal that they pass off to make daily entries to help each other out with their problems. Although most of the time it seems that it's Meiko counseling Miki.
An aspect that hasn't been discussed so far is art work. Shojo manga tend to use the typical anime art style of characters with over sized eyes, not that I'm saying all facial features in shonen manga are proportional. Though both kinds of manga use the spiky hair style made famous by Dragon Ball Z. Both also use cute mascots not unlike Pokemon.
Where it really counts though shojo and shonen manga are the same. Both have the passion of the author sewn into the pages along with the love and adoration of all their loyal fans.