JPEG
A photo file that throws away detail the eye misses, so the file stays small.
See it in Image glitch. Open JPEG corrupt and lower Quality until the blocks show.
In plain words
Photos hold millions of pixels, which makes large files. JPEG shrinks them by dropping fine detail and subtle colour changes that eyes rarely notice. It works on small squares of the picture, one at a time. At high quality the loss is hard to see. At low quality the squares show up as blocks and smudges.