What is the difference between genetic modification and conventional breeding?

Traditionally, a plant breeder tries to exchange genes between two plants to produce offspring that have desired traits. This is done by transferring the male (pollen) of one plant to the female organ of another. This cross breeding, however, is limited to exchanges between the same or very closely related species. It can also take a long time to achieve desired results and frequently, characteristics of interest do not exist in any related species. GM technology enables plant breeders to bring together in one plant useful genes from a wide range of living sources, not just from within the crop species or from closely related plants. This powerful tool allows plant breeders to do faster what they have been doing for years – generate superior plant varieties – although it expands the possibilities beyond the limits imposed by conventional plant breeding. (Source: ISAAA.org)

Whatever technique is used, the genome of the new variety is different from the parents, but convention dictates that this is not considered to be genetic modification, the term being reserved for the products of r-DNA technology. GM technology aims to produce new varieties by adding (or modifying the expression of) specific genes known to control particular traits. 

GM is more targeted (only a few genes carrying known functions are inserted in the recipient genome) and more rapid (bypassing the multiple cross generations needed by traditional breeding).  

It also allows plants to be used to produce molecules which could not be obtained otherwise, such as vaccines or bio-plastics.

Where conventional techniques are effective, they will be used, but genetic modification allows a wider range of useful traits to be incorporated into a given crop.