SHONEN MAGAZINE
Weekly Shōnen Jump (週刊少年ジャンプ), is one of the longest-running magazines in Japan. It has also been published in the US, Canada, Germany, Brazil and Sweden.
This magazine has published several manga series which are very popular right now.
Shōnen Magazine (少年マガジン) follows the list. The magazine is published by Kodansha, and it has 4 million followers every week. 
Shōnen Sunday (週刊少年サンデ) is also a very popular shonen magazine, published by Shogakukan. It has published three of the most popular manga series from Rumiko Takahasi: Inuyasha, Ranma 1/2 and Urusei Yatsura.
SHOJO MAGAZINE
The magazine that represents shojo the better would be Ribon (りぼん), a monthly Japanese magazine for girls roughly 9 to 13 years old.
SEINEN/JOSEI MAGAZINE
Since these variety of manga is targeted to an older population, it's not published as much as other categories. One of the most important seinen/josei magazines is Young Magazine (ヤングマガジン), published weekly by Kodansha. Some of the mangas that have been published in this magazine and that you might now are: Chobits, xxxHolic or Akira.
