Masters and Johnson (two groundbreaking s*x therapists) coined the term "s*xual-response cycle" to mean the sequence of events that happens to the body when a person becomes s*xually aroused and participates in s*xually stimulating activities (intercourse, masturbation, foreplay, etc.). The s*xual-response cycle is divided into four phases: excitement, plateau, orgasm and resolution. There is no distinct beginning or end to each phase -- they're actually all part of a continuous process of s*xual response. Keep in mind that this is a very general outline of what happens to each of us as we become s*xually aroused. There is much variation among individuals, as well as between different s*xual events.
Both men and women go through all four phases, except the timing is different. Men typically reach orgasm first during intercourse, while women may take up to 15 minutes to get to the same place. This makes the likelihood of simultaneous orgasm during intercourse a rare event.
Phase One: Excitement