This will lead to rays which are less parallel to the optical axis being focussed differently from those which are parallel or almost parallel to the optical axis. This means that for some points the object will always be blurred as while you can focus the light for some of the rays you cannot focus it for all of them. This leads to the problem of field curvature. As the distance from the centre of the lens to the focus point is constant then the image described by the lens is going to be a curved surface not a flat one. This leads to distortion.
Distortion is where parts of the image are magnified more or less than others.
These can commonly be seen on TV's and computer monitors. The majority of aberrations become greater the further away from the optical axis you go. Due to this it is quite common to reduce the aperture of the lens. The aperture of the lens is the part of the lens you actually allow light to enter. If you reduce the aperture then you block rays which would strike the edges of the lens from entering.