I'm starting to read Louann Brizendine's "The Female Brain" (saw it at Border's and couldn't help myself). She's a neuropsychiatrist, graduated from Yale and UC Berkeley, taught at Harvard Med School - hence, I figure she knows her stuff.
According to Brizendine, there is scientific proof that women's brains are wired waaay differently than men, and that there are actual biological tendencies at work - like the need to nurture, what a woman looks for in a potential mate, etc.
I really can't argue with science, but I personally think our society/culture also has a lot to do with how men and women behave - and Brizendine agrees (she also mentions in her introduction that she was active during the second wave of the feminist movement, so at least I know she's coming from somewhat of a good place).
But back to the scientific stuff, apparently women put their efforts toward creating close relationships and bonds with people; men are more interested in being at the top of their social group. Women are also more adept in communication, speaking more often and faster than men, who can go for hours without verbal exchanges. Hormones play a huge role, as well - especially when a girl enters puberty and estrogen levels surge. Even during a woman's normal menstrual cycle, one week she's sharp, witty and on top of the world - the next, she's slower, irritable, and her self-esteem hangs by a thread. Most women can relate: all of the sudden, your partner thinks you're ugly, your friends hate you, and why did your boss hire you for this job anyway? Some men are quick to label this as completely irrational female craziness - but it's biology.
Now, I know a lot of feminists loathe the word "biology" - it's the anti-feminist explanation for sexism, a way to generalize men and women and hold them to standards for what each sex should be doing. I think completely relying on biology to tell us about men and women is ridiculous, but so is completely ignoring it. As humans on Earth, we don't operate in a vacuum. Culture and society have changed incredibly since the Stone Age - there are certain biological tendencies and behaviors that are probably still present, but maybe we don't quite need anymore for basic survival.
Men and women are inherently different - I will argue that til I'm blue in the face - but where my feminism comes from is the belief that one is not better than the other.