There are two main forma of manga in my opinion: liscensed and unliscensed. Liscensed manga is the manga you see in bookstores in the 'Graphic Novels' section. Two main companies that liscense manga are TokyoPop
(
http://www.tokyopop.com ) and Viz Communications (
http://www.viz.com ). Unliscensed manga is manga that (obviously) has not be liscensed by a translating company like the two mentioned before. You can get some unliscensed manga online at different scanlation (scans +translation) sites where teams put the manga in downloadable form online or on IRC (Internet Relay Chat).
In manga (like in books) there are different genres. Shoujo (which means 'girl' in Japanese) is aimed at girls. Shounen (which means 'boy' in Japanese) is aimed at boys. Seinen is aimed at older audiences and has more violence or something (I don't know much about it I try to stay away from it). Mature shoujo is aimed at older girls/women sometimes and usually has sexual references/and or situations (again, something I personally stay away from). Shoujo-ai/Shota is about homosexual girls and Yuri is too except yuri has more 'adult situations' (I recommend that you stay FAR away from it and the next two-shounen-ai and yaoi). Shounen-ai is like the boy version of shoujo-ai (boys liking boys) and yaoi is the boy version of yuri (boys liking boys w/ adult situations). Hentai is like porn drawing and ecchi is manga w/ erotic situations (again, I recommend that you stay away from those two). The only good ones that I mentioned are the first two, shoujo and shounen. If you want to read a nice unliscensed manga online I would suggest you going to dgt.glowie.com (w/out the www.). That scanlation group has some cool projects there to read like Shishun Shiteru Kai!, Aoi Spice, and Mekakushi no Kuni. Some cool liscensed manga that you can find in stores are DNAngel, Tokyo Mew Mew, and Cyber Idol Mink. For more manga recommendations and or manga info you can pm me, email me (check my profile for my email address), or check this thread here to see what everyone else is reading at the moment
http://www.christiananime.net/showthread.php?t=8724&page=1&pp=10 . I hope that you find something good to read that you'll like!
" 'Tis strange-but true; for truth is always strange, stranger than fiction." ~Lord Byron
"When you see a fork in the road, take it." ~Yogi Berra