Selective breeding is the process of choosing only some animals to breed from a group. Imagine a farmer who has 25 cows. He wants to breed them for the next generation. But he wants big, fat cows that give lots of milk and meat. So he chooses the biggest, fattest bull and the milkiest cows and he selectively breeds those. He does not breed his skinny cows or aggressive bulls. So the next generation of his herd of cows is bigger and fatter and milkier and more gentle.
Then, from that new generation he selects the biggest, fattest bull and the milkiest cows and he selectively breeds those....
Now repeat this many times. Generation after genration of farmers select the cows and bulls which have the charecteristics that they want until we have the modern cow, a very fat and milky peaceful animal which produces the most meat and milk.
(Selective breeding is mankind's use of Natural Selection, a process of evolution.)
Eugenics is the study and practice of selective breeding applied to humans, with the aim of improving the species. In a historical and broader sense, eugenics can also be a study of "improving human genetic qualities." Advocates of eugenics sought to counter what they regarded as dysgenic dynamics within the human gene pool. Specifically, in regard to the continuation of congenital disorders and factors impacting overall societal intelligence relating to the heritability of IQ.
Eugenics was widely popular in the early decades of the 20th century, but has fallen into disrepute after having become associated with Nazi Germany. Since the postwar period, both the public and the scientific communities have associated eugenics with Nazi abuses, such as enforced racial hygiene, human experimentation, and the extermination of "undesired" population groups. However, developments in genetic, genomic, and reproductive technologies at the end of the 20th century have raised many new questions and concerns about the meaning of eugenics and its ethical and moral status in the modern era.