What is physically happening when a women orgasms? 

Blood heads straight to the vagina and clitoris. The walls of the vagina  secrete beads of lubrication that eventually get bigger and flow together. As the woman become more turned on, blood continues to flood the pelvic area, breathing speeds up, heart rate increases, nipples become erect, and the lower part of the vagina narrows in order to grip the penis while the upper part expands to give it someplace to go. Nerve and muscle tension builds up in the genitals, pelvis, buttocks, and thighs until your body involuntarily releases it all at once in a series of intensely pleasurable waves.

The orgasm is the moment when the uterus, vagina, and anus contract simultaneously at 0.8-second intervals. A small orgasm may consist of three to five contractions; a biggie, 10 to 15.  A recent small-scale study at the Netherlands' University of Groningen found that areas involving fear and emotion are actually deactivated during orgasm.

This a straightforward biological answer to this question. Out of all the questions asked the topic of female orgasm is number one in peoples minds so tomorrow we will continue with:

Why can women have multiple orgasms and men can’t?