Using selective breeding techniques, people have created everything from variegated roses to giant pumpkins to strains of wheat with twice the yield and very high disease tolerance. In the same way, you can take chickens, analyze their eggs and find chickens whose eggs contain less cholesterol. Then you can breed them to create a strain of low-cholesterol chickens. You can select on any trait you can detect and selectively breed members of the species that do well on that trait. Genetic engineering techniques now allow scientists to insert specific genes into a plant or animal without having to go through the trial and error process of selective breeding.
Genetic engineering is therefore extremely rapid compared to selective breeding.
You can read the HSW page entitled How Cells Work to learn about DNA, genes and gene splicing. The techniques are now standard and genes can be spiced very easily. There are a variety of ways that scientists now modify plants and animals with genetic engineering techniques. For example, there is a widely used herbicide called Roundup, made by Monsanto. Roundup kills any plant that it touches.