Viewed from a mere technical point of view: In photography you process a bunch of light that's passing through your lens into a picture that you can hang on you wall for instance. Post processing itself can be explained in different ways. Most commonly it's referred to as the adjustment you do after the the shutter closed, but basically that's a little part of the processing I mentioned in the first place.
When you adjust your aperture you change what the final image will look like. If you do an adjustment in post processing, lets say in photoshop for the digital photography, your adjustment might be very likewise but in a different stage in the process.
For me it is very important to get the shot as good as possible when capturing. Not at all because post-processing would be wrong, as some people might claim, but because of the experience that for every step in the processes from light to a print on the wall there's an optimal moment to take care of a certain part.
To put that in context: You've to take care of the composition when shooting, because there isn't barely anything you can do about it when post processing. Contrary to that I use quite some colour filters when converting my shots to black and white. Simply because It's way easier to do in post processing I apply the filters in processing instead of carrying a bunch of colour filters with me when shooting en screw them on at the moment I take the capture.
Besides that, people who think a jpeg from their camera is un-processed and they don't apply any post processing on their computers because they are 'purists' are just foolish. They certainly do not have a clue what's going in in their camera.