What turns people on differs. But the sexual response proceeds through the same predictable stages:
Desire/lust
Arousal often starts with desire or lust. You long to masturbate: you go to your room, close the door and caress yourself. Or you look at the breasts of your girlfriend and can almost feel them under your hands. Taking initiative to make love is only a small step.
Excitement
Excitement can be triggered by physical or psychological stimulation. One of the first obvious signs of arousal is your vagina getting wet or your penis becoming erect. Normally you also notice that your heart rate increases. With women the nipples can get erect. This stage can last from a few minutes to hours. After the initial sight, smell, touch or taste you need additional stimulation to go through the next stages of arousal. Otherwise the excitement is lost and regained many times without progression to the next stage.
Plateau-phase
After the first quick excitement the sexual tension normally levels off. This is why it’s called the plateau phase. With girls this stage normally lasts longer than with guys, especially during lovemaking. That’s why guys often have to put their mind on postponing orgasm, to make sex last longer, while girls may have the feeling there is not enough time. Some girls have difficulty getting past the plateau phase.
Orgasm
With orgasm there is a rapid and pleasurable release of tension. Because people differ in the time they need to climax lovers seldom have simultaneous orgasms.
Resolution
The blood rushes away from the vulva and penis, the heart rate slows down and breathing becomes normal again. The sex flush disappears and the nipples, breasts and testicles regain their normal size. The body returns to its pre-excitement state.