http://health.howstuffworks.com/sexual-health/sexuality/brain-during-org...

Although the reasons for having sex of any kind are varied and complex, reaching orgasm is usually the goal. Because we're all so different, coming up with a universal description of an orgasm is impossible. The one thing that most people can agree on is that it's an incredibly, intensely pleasurable experience.

So what is it? When in doubt, go to the dictionary. The Oxford English Dictonary defines an orgasm as "a sudden movement, spasm, contraction, or convulsion [...] a surge of sexual excitement." Merriam-Webster gets more descriptive, stating that it's "an explosive discharge of neuromuscular tensions at the height of sexual arousal that is usually accompanied by the ejaculation of semen in the male and by vaginal contractions in the female." The famous sex researcher Dr. Alfred Kinsey once said that an orgasm "can be likened to the crescendo, climax, and sudden stillness achieved by an orchestra of human emotions ... an explosion of tensions, and to sneezing" [source: Geddes].