JPEG / JFIF

JPEG per se isn't a file format. The committee weren't focused on storing the data from their compression algorithm as files, they were thinking you'd transmit it to somewhere and it'd get decompressed and then used. So the actual international standard is completely mute about files on disk.

Early on people who did think we should store data in files wrote JPEG compressed data to the pseudo-standard TIFF. But TIFF is a complete mess, conceived as the minimal way to store output from a drum or flatbed scanner on a computer and thus permitting absolutely everything but making nothing easy - and its attempt to handle JPEG led to incompatible (literally, as in "I sent you the file" "No, my program says it's corrupt" "OK, try this" "Did you mean for it to be black and white?") implementations. There were then a series of Adobe "technical notes" for TIFF that try to fix things, several times attempting a fresh start with little success.

JFIF is the "real" name for the file format we all use today, and it's basically where the IJG comes into the picture. Instead of TIFF's mess of irrelevant or nonsensical parameters you've got the exact parameters needed for the codec being used, and then you've got all this raw data to pass into the decoder. And there's this handy free library of code to read and write the files, so everybody just uses that.

So initially the IJG are great unifiers - instead of three or four incompatible attempts to store JPEG data in a TIFF you get these smaller and obviously non-TIFF JPG files and either the recipient can read them or they can't, no confusion as to what they mean. But then they proved (and libpng followed them for a while) incapable of grasping what an ABI is.