Distortion is an optical effect caused by lens structure. There are three types of distortion: barrel, pincushion, and mustache, which is a mixture of barrel and pincushion.
Barrel distortion is the most commonly seen type of distortion and is found in normal to wide angle lenses, with more pronounced examples seen in the widest focal lengths. Picture an image produced by a fisheye lens and you will have an image in mind that features strong, very exaggerated barrel distortion. It should be mentioned that not all wide angle lenses suffer barrel distortion - there is a specific type of lens called ‘rectilinear’ which has little to no distortion
Pincushion distortion is less common and can be seen in older or low-quality telephoto lenses. Rather than lines bowing out as in barrel distortion, they converge inward toward the center when pincushion distortion is present.