Compression reduces the number of bits used to represent each pixel in the image. The algorithms used to compress the images introduce, to a lesser or greater extent, visible artifacts. In general, the more complex algorithms introduce fewer of these artifacts. The processing power available to us increases with time, so more sophisticated algorithms always are being introduced into the codec products. This has to be done in a way that uses realizable computation, and has a short processing delay. The processing should not distort the image so that visible artifacts are generated.
As the level of artifacts decrease, so does our acceptance of the distortions. Once we were happy to watch monochrome television; now ...