In determining gender, babies born with a penis are male and babies born with a vagina are female.
In more technical biological terms, boys have an X and Y chromosome, and girls have two X chromosomes. This is why males are more prone to sex-linked diseases, such as colorblindness. While females need both parents to either have or be carriers of a disease, men only need one parent to have the disease in order to inherit it.
Males and females have a different skeletal structure. Women have a shorter head, broader face, smaller chin, shorter legs, and longer trunk. The first finger of a man's hand is usually shorter than the third; in women the reverse is true. Men's teeth last longer than women's.
Women have a larger stomach, kidneys, liver, and appendix than men. Men have larger lungs.
Women's bodies can do 3 things men's can't -- menstruation, pregnancy, and lactation.
Women's blood contains more water and therefore tires more easily and is more prone to faint.
Men tend to have higher blood pressure but a slower heart beat.
Women can withstand higher temperatures better than men because their metabolism slows down less.
Dobson, James. "Physical Differences Between Men and Women." My Family Talk. Dr. James Dobson's Family Talk, n.d. Web. http://drjamesdobson.org/Solid-Answers/Answers?a=ff773023-2693-410d-b9e1-662f6985be4e