One big difference is the <em>sentence ending particles</em>.
For example,
(In a casual setting)
zo, for men, as in Okashii-zo. (Something's wrong.),
noyo, for women, as in Okashii-noyo.


Also, men tend to use words which originate from China, called 漢語（Kango) whereas women tend to use Wago(和語）, Japanese before the addition of foreign elements.

This is a very very good question.
I'm confused too about how to understand the genre of the writer in Japanese.
Thanks in advance.

My first Japanese lesson with my Japanese teacher.
You need to learn hiragana.Write it.
I asked to him can i write  sentences written by my penpal.
I've done the work but i din't told to my teacher i was studying with a guy.My teacher had  read the sentences he said oh you are studying with a girl. I said no. <img src="http://www.italki.com/fckeditor/editor/images/smiley/msn/teeth_smile.gif" alt="" />
I asked to him why he tought this sentences were written by a Japanese female.
He said about the sentence structure we can understand if the writer is a female or a male. ^^
A day I received an e-mail but i din't know if was written by a guy or a girl the person was hiding the sex in the profile my teacher said this is a girl. He was right. ^^

i don't talk to men simply because it wont end up fighting in the end. i rather talk to girls