Viewing 3D is commonly called Stereoscopy.
Whether you're watching 3D porn, or a regular 3D movie, the concept always stays the same: using different techiques (look at the menu 'how-to's => types of 3D' to find more) a slightly different image is showed at both your left and your right eye. Your brain converts both images, and "creates" the illusion of 3D.
The image shown to both your left and right eye is recorded with a camera with two lenses, the lenses are seperated by a space equal to the median distance between the eye's of a "standard" human.
This works for most human beings, but since the space between your eyes is almost never exactly the same as the mean distance for the full population of humans, some people might experience some nasty side-effects from 3D viewing (things like nausea). For those people, 3D porn will always remain a dream (luckily, 2D porn is still available)