Chromatic aberration is a defect where the different colors of light focus into different places, creating a red image in one spot, blue image in a different spot and the green image in a third.
This effect is caused because red light is not the same as green light, so the angle at which it is reflected when it passes through the lens is marginally different. Depending on the complexity of the lens and the number of lens elements, the difference in angles between the colors can become extremely large, producing images which are unusable.
Chromatic aberration is fixed using a number of complex techniques. Currently, lens makers will use a lens element to make a certain color focus at a set distance, and then use another lens element to focus a different color at that length, and so on until most of the colors are focused in roughly the same place.
Come is an effect where stray light rays are directed off the focal axis, giving images a strange out of focus blur or streak. It is noticed most often when using lens with large aperture, because the light is less focused with these lenses.
Coma is extremely hard to fix. It is generally fixed by using lens elements with a smaller diameter, however, this cannot be done to lenses with large apertures. With these lenses, special formulas are used when making the glass, and oil coatings are put on the lenses.
Geometric distortion, also known as pincushion or barrel distortion, is where the image is squeezed or crushed due to the circular properties of the lenses. Geometric distortion is the hardest defect to detect in lenses, it cannot be noticed unless taking pictures of charts with perfectly straight lines.
Because lenses are circular in shape, all will have geometric distortion to some degree. The smaller the radius of the lens, the less the distortion will be. Generally, geometric distortion is corrected with software that has been programmed for that particular lens, rather than changing the glass.