This data is not redundant data like we saw before, but actually significant data from the original picture. The idea is that we won't miss the data we can't see. Just enough image date is eliminated so that we still perceive an image with the same quality as the original. People, however, have different abilities to perceive image quality so an image saved with a certain amount of compression may appear high quality to one person and moderate quality to another. Once saved, there is no way to retrieve the original quality image without scanning the original picture again.
Artifacts are features introduced to the image by the mathematical rules used in compression.
These occur when the elements used by compression to make up the image become large enough to see. Or subtle banding or color shifts from the original occur. Often, the non-lossy format acts like a one way street. You can scan and save an image with acceptable quality but you can't open the image again to editing without losing some quality. Each time through the editing cycle, the distortions crated by repeated compression of an already compressed image and its attendant artifacts degrade image quality further until it is unacceptable.