Spherical Abberration:
In optics, spherical aberration is an image imperfection that occurs due to the increased refraction of light rays that occurs when rays strike a lens or a reflection of light rays that occurs when rays strike a mirror near its edge, in comparison with those that strike nearer the center. It is often considered to be an imperfection of telescopes and other instruments which makes their focusing less than ideal due to the spherical shape of lenses and mirrors. This is an important effect, as spherical shapes are much easier to produce than aspherical and so most lenses have spherical shapes.
The effect is proportional to the fourth power of the diameter and inversely proportional to the third power of the focal length, so it is much more pronounced at short focal ratios, i.e., "fast" lenses.
Geometrical Abberation:
Straight lines, or light paths - orthogonal to the wavefront surface. Disturbance of rays caused by wavefront errors manifests as angular deviation of the aberrated ray, producing linear ray deviation in the image plane, usually either from Gaussian image point, or from best (diffraction) focus.