Thankfully, the newer image editors offer a Preview window where the effects of JPEG compression on image quality can be judged prior to saving the file. In this sense, the JPEG format is not considered archival. In the JPEG image compression processes, the computer works by throwing away parts of the image that it thinks are unimportant. It makes this decision based on knowledge about what the human eye can see or perceive. Information in the image that the eye is not as sensitive to is removed or compressed.
Information used by the eye to perceived image quality is retained.
What is kept and what is saved is determined by a complex mathematical equations. One of the drawbacks of using this method is that human visual perception is varies from person to person. An image I think has high quality you might think has marginal quality. A computer cannot make this decision for you, so when saving an image with JPEG compression you will have to use visual judgment as to what is an acceptable level of compression. The series of mathematical steps used in this compression technique first divides the image up into square blocks.