Distortion is a type of optical aberration that happens when there is a variation in magnification across the visual field.

 

Barrel distortion results when a lens magnifies slightly more at its thickest part (along the optical axis) than it does at its edges; the image of a square shape would thus appear to have bulged-out sides. Barrel distortion is associated with wide-angle lenses and only occurs at the wide end of a zoom lens.

 

Pincushion distortion comes about when magnification increases away from the optical axis causing image lines, both vertical and horizontal, to bend in toward the center of the image.

 

Neither kind of distortion affects resolution; it simply means that the image shape doesn't correspond exactly to the shape of the object.