Although an answer has already been accepted, I will allow myself to add one more unsaid argument.
Post-processing is a very important step, especially in the field of digital photography. The point of post-processing is to a) correct photographer's errors (horizon line, exposition), b) enrich a photograph with specific mood by altering its color gamut or introducing cropping, c) make it look natural. At least the last point requires some explanation.
A digital camera has a different working principle than a human eye. It captures light linearly (light is captured by a counter by every pixel on the sensor) whereas human eye sees more detail in light areas and less so in darkness. We say that a human eye has a non-linear color gamut. This is why mid-day shots appear so burnt-out when shot with a digital camera. White balance is another similar story where camera has to apply algorithms to deduce the correct white balance. Human eye adapts automatically. One could say that post-processing is a built-in option in human brain.
To make a photograph look "natural", we alter the light curve, change white balance, enrich saturation and play with a number of other settings. If this is done qualitatively enough, a person unfamiliar with photography will not notice anything. For him/her this will be a standard, maybe "good-looking", photo.
Post-processing is fully justified in cases when it remains unseen (like in the photo by Ansel Adams quoted in the answer by Michael Clark) or when it adds artistic effects. When neither of these applies, when people can not see artistic in the photograph, they tend to seek for the natural. I am afraid that when you hear people tell you your photo has been photoshopped it might indicate that you have altered it far too much so that it looks neither artistic nor natural.