There is a pretty good compression artifact article on wikipedia:
http://en.wikipedia.org/wiki/Compression_artifact

that should give you a base-line of understanding of how they come about and what they look like.

http://en.wikipedia.org/wiki/JPEG

That's specifically for JPEGs though MPEGs show quite the same thing.

http://www.cambridgeincolour.com/tut...imagetypes.htm

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