Post-processing in digital photography usually denotes the process equivalent to developing the film in film photography (which can be a complex process where multiple choices can be made that affect the result). Sharpening, noise reduction, brightness, contrast and so on. These are an integral part of the workflow - nobody would simply publish raw images straight from the camera (turning the raw sensor data into a JPEG involves multiple choices that someone has to make - it's either the photographer or the engineers that wrote the default conversion algorithms built in the camera.)
There are various movements in photography. You still need to use some techniques while doing some photographies but usually for straight photography, it ends when you press the shutter.
But I suppose that Henri Cartier-Bresson expressed it in a better way:
"Of all the means of expression, photography is the only one that fixes a precise moment in time. We play with subjects that disappear; and when they’re gone, it’s impossible to bring them back to life. We can’t alter our subject afterward.... Writers can reflect before they put words on paper.... As photographers, we don’t have the luxury of this reflective time....We can’t redo our shoot once we’re back at the hotel. Our job consists of observing reality with help of our camera (which serves as a kind of sketchbook), of fixing reality in a moment, but not manipulating it, neither during the shoot nor in the darkroom later on. These types of manipulation are always noticed by anyone with a good eye." Henri Cartier-Bresson - "American Photo", September/October 1997, page: 76
Post-processing in the sense I referred to usually has the goal of replicating, not altering, the moment of firing the shutter. Cameras (film OR digital) do not see the way the eye sees; the image straight from the camera (which, as I said, does all sorts of automatic processing to be able to create the image in the first place) is rarely a faithful presentation of what the photographer saw -- not to mention the subjective mood of the moment that the photographer is trying to capture.
I think it's a very small minority of photographers indeed that are purist enough to consider things like cropping, levels, and white balance "altering the subject" in some reproachful way.
