Posterization is an effect that uses a limited color palette or range of tones. A posterized image doesn't have subtle gradients or continuous tones; no smooth transition from one color to the next. Posterization can be intended or unintended.
On the other hand, images may be intentionally posterized to give a photographic image a stylized line art or impressionist painting appearance. Once a complicated darkroom process, today intentional posterization can be done in just a few clicks in Adobe Photoshop, Corel Photo-Paint, or other image-editing software.
In the sidebar, the two images on the left half are a digital photograph and a gradient fill with no posterization. After posterization, the banding or color shift is obvious in the images in the right half.