Our bodies go through a number of changes during sex. Understanding these changes helps us understand both our sexuality and that of our spouse.
Most scientific folk divide these sexual responses up into four phases - arousal, plateau, orgasm, and resolution. This article looks at women, you can read about men here.

AROUSAL - Blood begins to move to various parts of the body. The breasts begin to enlarge and nipples may become erect. The outer labia lay flat and the inner labia begin to swell and darken in color. The clitoris swells and becomes more sensitive. The vagina begins to lubricate, and the uterus starts to move up and away from the vagina. Note, it was previously believed that the uterus doubled in size during arousal; in fact, it does not change size, but simply moves up into the body.

PLATEAU - With further stimulation, the clitoris pulls back further under the hood. The inner and outer labia will darken, becoming quite dark or even red just before orgasm. Women who have been pregnant have a better blood supply to the genitals, and their labia will darken more than before they had children. The vagina expands and elongates, ballooning out in the deepest two-thirds. The outer one-third of the vaginal wall thickens (due to vasocongestion) and contracts, forming what is called the "orgasmic platform," and the uterus elevates fully. Like men, heart rate and blood pressure increase, and a skin flush may appear on the chest, neck or face (actually these "sex flushes" are more common among women). Breathing increases, and soft vocalization may occur. If position allows, the hips may be moved in a rocking motion which thrusts the genitals up and down. If this motion is exhibited, it will increase as orgasm gets closer, possibly becoming rather dramatic. The legs may be opened farther and/or repositioned as orgasm approaches.

ORGASM - At orgasm the outer one-third of the vagina contracts. The uterus and rectal sphincters also contract. There may be foot spasms or contracting facial muscles. Muscles tense, and breathing, heart rate and blood pressure rise further. Some women may release fluid (see female ejaculation). In women the length of an orgasm is affected by the type and duration of stimulation both prior to and during orgasm.

RESOLUTION - A fine perspiration may cover the body. The external genitals gradually return to normal size and position, as do the breasts. The vaginal wall thins and collapses back into its resting state. The uterus drops back into place. The cervix opens slightly and drops into the pool of semen left at the entrance of the cervix (unless you used a condom, of course). Breathing, heart and blood pressure return to normal and muscles relax. If stimulation is continued or restarted, resolution is delayed or stopped, and more orgasms are possible. It takes the woman's body far longer to return to "normal" than the man's, easily a half-hour or more.