If you ever tried to save photos to JPEG format, you probably know how bad JPEGs can look. Decades of domination of this format has led to most internet users having an allergy to effects called “JPEG artifacts.”
No doubt, no one wants to share such a mess with someone else. If you ask me what exactly is wrong with the image, I’d probably say that in general it looks very close to the original: no significant color shift, all objects are distinguishable. However, I’d say there are two major problems:
Soft gradients look like separate blocks or bands. That is why it is called color banding. This is especially noticeable on the sky, but you can see the same on the ground.
There are parasite color gradients around sharp objects (look at the roof), and other tiny elements. This effect is called ringing.
So, the left image is a reference with perfect quality. The center image is heavily compressed, but only has ringing artifacts, while the image on the right has only color banding. All three images are actual JPEGs, crafted with the unmodified libjpeg library, no Photoshop involved.