Aberrations, or optical imperfections, exist because no lens is optically perfect. Almost all lenses show aberrations from the ‘perfect’ image.
Vignetting is sometimes also called 'corner shading'. It's where a picture is darker at the edges than at the center. It's a lens aberration which you tend to see mostly in older and cheaper lenses and it can be corrected in software. Sometimes vignetting is used as a creative tool to product a vintage look or to improve the picture's composition.