What is AVIF?
AVIF is a still image squeezed with the AV1 video codec, and it compresses harder than anything else in wide use - frequently half the size of a JPEG that looks the same. It handles transparency and high dynamic range. Support is good in browsers and patchy everywhere else, which is usually why people convert out of it.
What is JPG?
JPEG is the format photographs live in. Its compression is lossy - it discards detail the eye is bad at noticing, and discards a little more every time the file is re-saved - which buys enormous size savings on photographic images and makes a mess of screenshots, logos and text. It has no transparency.