A PNG is a kind of picture that's mostly used for icons and logos. PNGs aren't the smallest, but they let you have transparency in a picture and they're very good quality – no details are ever lost in a PNG.
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A JPG is a kind of picture that's often used for photos. Pictures saved as JPEGs don't take up very much space, but their smaller size is because they're a little worse quality – most of the time small details will be lost.