In simple terms, post processing includes all the processes that are used to transform an image from what was captured by the camera to something closer to what our eyes see or to change the image. Image editors use software like Adobe Photoshop for post processing works, but it can also be done without using Photoshop.
It is true that good photographer will be able to get more out of his or her camera than an average person who is not well versed with high-end digital cameras but post processing cannot be avoided as it has become an integral part of what makes a professional photographer these days. In the simplest of terms, post processing is nothing but using computer software to make changes to an image.
Some of the shooting techniques rely on post processing and understanding these techniques can help you know more on post processing. These techniques are used by photo editing services knowing that they will be post processing the shots.
A good amount of both professional and hobbyist photographers shoot in RAW image format. When photographers use the RAW mode, they know that the images need post processing. RAW images are like the raw hamburger meat that need to be cooked, whereas the JPEG images are like the fully cooked hamburgers. It is true that you can add things to it to change flavors, but you will have only few options as the cooking is already done.
Even if you have selected the settings that are perfect for the particular shot, you need to do some work the raw data provided by the sensors of the camera. While capturing images in JPEG format, the camera captures the image in a predetermined way that cannot be reversed later, but while shooting in RAW format, the photographer or the image editor will have the do the necessary processing with software. This editing of RAW images on a PC can be considered a form of image post processing.
High Dynamic Range (HDR)
High Dynamic Range (HDR) shooting is another photography technique that knows ahead of time that the pictures will be post processed. Images that are captured with a camera are not even close to what you see with your naked eyes. It is true that no camera is capable of capturing the range of light that human eyes can handle. There is a very big difference between the brightest and darkest parts of, say a sunset, that the camera can handle and capture clearly.
When there is a very big difference between the dark and bright (Shadow and highlight) parts of the scene, it is usually referred to as having a high dynamic range. Now you may have understood how limited the best cameras are in capturing the full dynamic range of images. This is why HDR image capturing makes use of post processing techniques.