JPEG files use a technology that compresses the image size down to save disc space.  JPEG image files are quickly becoming the most common format for photo storage.  All digital cameras produce JPEG files.  The downside to JPEG files is that repeated editing and saving of JPEG image files will reduce the image quality.  With JPEG, you should always keep a copy of the original unedited images for your archival copy of the image.  Opening the files or copying the files will not affect the quality.  But when you edit the JPEG file and then resave it, it must recompress the image and each time that process takes place the image quality suffers a little.  We use the minimal compression / highest quality setting on all JPEG images.  It is virtually impossible for all but a professional expert to see a significant difference between a TIFF image and our JPEG images.