Compression artifacts are typically the result of quantization in lossy data compression. Where transform coding is used, they are typically in the form of one of the basis functions of the transform coder's transform space.
Compression artifacts in picture coding
Where the DCT image transform is used, for example, they are often 8x8 pixel squares, containing a stripe or "checkerboard" pattern.
Where predictive coding of motion pictures is used, as in MPEG-1, compression artifacts tend to remain on several generation of decompressed frames, leading to a "painting" effect being seen, as if the picture was being painted by an unseen artist's paint-brush.
Where motion prediction is used, as in MPEG-2 or MPEG-4, compression artifacts tend to move with the optic flow of the image, leading to a peculiar effect, part way between a painting effect and "grime" that moves with objects in the scene.
Errors in the bit-stream can lead to errors similar to large quantization errors, or can disrupt the parsing of the data stream entirely for a short time, leading to "break-up" of the picture. Where gross errors have occurred in the bit-stream, it is not unknown for decoders to continue to apply 'painting' updates to the damaged picture, creating "ghost image" effects.
To stop the build-up of compression artifacts, most compression systems occasionally send an entire compressed frame without prediction or differencing. In MPEG picture coding, these are known as "I-frames".