"The first artifact is called Macroblocking. Areas with slight differences in color cause nearby block and macroblocks to be tinted differently. Background areas (e.g., images of blue sky or green fields) and dark areas in the image are usually affected the most when the bandwidth is too low. The artifact makes the image appear digitized and unnatural."
"The second artifact is called Mosquito Noise. This artifact appears as specs around edges of objects. With a bit of imagination, one could imagine a swarm of mosquitoes around people's faces, channel logos, or even subtitles. The more compressed the image, the more noticeable mosquito noise becomes."