These can be either blockiness or mosquito noise and are usually seen in areas with high contrast, such as near lines. If in doubt, refer to some of the images with this tag.
This tag should be used for images where these artifacts are noticeable when viewed normally, without zooming in or enhancing the contrast.
Example
An image devoid of artifacts (sourced from post #8012139)
The same image, but with visible artifacts (sourced from post #8012103)
