The image's color quality is affected. This compression is called 'lossy' because some image information is deliberately discarded in order to reduce filesize. For instance, an uncompressed image of blue sky would require much space on disk to store hundreds of shades of blue. Since our eyes can't discern all those different shades, the number of 'blues' can be greatly reduced. This allows a JPG ' copy' to look almost exactly like an original, yet occupy only a fraction of the file space taken by an original.
Variable compression allows a wide range of quality and filesize.
Unlike lossy compression, when an image utilizing lossless compression is opened, every single pixel is reset to its exact original color. While stored on disk, everything is in a form of shorthand. But when you unzip and open of the file, all your words are exactly as they were. Because lossless compression does not discard any image data, it cannot compress a file nearly as much as with lossy compression. This refers to the stair-step effect created when square pixels are used to simulate round curves.