What is HEIC?

What an iPhone saves photos as. It isn't really an image format so much as a container holding HEVC video frames - the same codec used for film, applied to one still. That gets a photo to about half the size of the equivalent JPG, and it's also why so little else opens one: HEVC is patent-licensed, so most browsers and a lot of desktop software can't read a HEIC at all. Big photos are usually stored cut into tiles that have to be reassembled.

What is PNG?

PNG is the web's lossless image format. It compresses without ever degrading the picture and has a real alpha channel, which is why screenshots, logos, line art and anything with sharp edges or text belong in it. The trade is size: on a photograph, PNG is enormous compared to JPG.