Ooo boy. Technically its a release of bodily fluids from both partners. And which heights the senses and makes you feel momentarily euphoric. That's pretty much the whole point of sex. If it wasn't there would be bout a quarter of society left today.
An orgasm is your bodies response to sexual satisfaction. Excessive blood flow to the genitalia making it even more sensitive to stimulation. It is an incredible feeling that really can't be described except through experience. But trust me on this. You will know when you have one.