My Definitions

Boy

I define a boy as a male who lacks maturity in a physical and mental sense.  When i say they lack physical maturity I mean they have not hit puberty yet.  Boys are also pretty immature when it comes to things such as kissing and other male and female interactions.  Also, boys are very dependent on their peers even if they think differently.

Girl

My definition of a girl is a young female who lacks physical maturity and isn't really worried about what other people think about her appearance.  Just like boys, girls begin their transformation from a girl to a woman during puberty.  Therefore any young female who has reached puberty yet is automatically defined as a girl.  Also unlike woman, girls aren't really worried too much about the way they look in front of their peers. 

Man

http://www.flickr.com/photos/cocoen/1065306490/

I would describe a man as a mature male who possesses the "manly" body features and is able to act as an adult when the situation calls for it.  One difference between a boy and a man is that they have slightly different views about females. Men view their counterparts with much more respect and unlike boys, men usually express their soft side around girls.  Also, men are more mentally mature than boys and usually think about the consequences to their actions.

Woman

http://www.flickr.com/photos/didmyself/6462306269/

I define a woman as a mature female who understands who she is and has developed what society calls "womanly" features.   She is mature enough to talk about sex and other serious matters without giggling or acting in a childish manner.  This applies to being a man too.  Also, she develops both an understanding of what society expects of her and a sense of independence.