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Posterization in a photographic image refers to a region of continuous gradual tone being replaced by a regions of fewer tones resulting in visible steps from one tone to another. This effect can be quite subtle, or very obvious.
Any tool in an image editing program that adjusts the histogram (such as Levels and Curves in Photoshop) has the potential to create posterization.

Below is the histogram of an 8-bit image.
8 bit histogram 
Now watch what happens when we make an extreme change using Levels in Photoshop.
8-bit Histogram post Levels 
This is what is called posterization. The comb effect. The missing vertical bars are caused because the original color intensities of 0 to 255 are being forced to spread over a larger range than existed in the original image. This causes the missing gaps to appear because there is no data available to fill those columns.

Below is a 16-bit histogram of the same image.
16 bit histogram 
Now watch what happens when we change a 16-bit image using the same extreme adjustment in Levels as before. 
16 bit Histogram post Levels 
Bingo, no missing data. This is because now there is a huge amount of data available to display the tonal range of the image and making extreme adjustments still leaves plenty of room for smooth tones. 16-bit images have up to 256 times more color levels as 8 bit images.
This is what an image would look like before posterization and after. 
No Posterization  Posterization 
Posterization can easily be seen in smooth graduations and it will ruin a image.

Always edit in 16-bit mode to prevent this from happening. 

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