Social Darwinism is an elitist ideology which asserts that individuals should work only for their own interests, and that society and government should reflect this idea by following the dictates of natural selection, thus maximizing the pace of human evolution. It generally advocates doing as little as possible to aid those who "fail" economically, even to the point of allowing them to die, while helping those who "succeed" by rewarding them still further, thus encouraging the propagation of "successful" genes.
It is consistent in many ways with the goals of eugenics, although eugenics might suggest (for example) offering affordable gene counseling and therapy to poor people, while social Darwinism would hold that natural selection should be allowed to do its work unassisted.
Creationists often try to use social Darwinism as proof of the bad consequences of Darwinism (a term they coined for "belief in the theory of evolution") and thereby as an attack on the theory of evolution, which (they argue) people should therefore choose not to believe in regardless of the evidence.
This is, of course, complete nonsense; it is an argument by association at best.
Darwinism, by itself, is not an ideology (a position advocating action) but rather a position or belief about the nature of the universe. As such, it is neither good nor bad, except in the secondary sense that accurate knowledge about the universe is a good thing to have. Further, changing one's beliefs about the universe does not affect how the universe actually operates; it only affects how well one is able to achieve goals through use of one's knowledge about the universe. Inaccurate beliefs are much more likely than accurate ones to result in ineffective actions.
It is a prime example of religious hypocrisy and intellectual dishonesty that many of the people who use social Darwinism as an attack on the theory of evolution nonetheless advocate social Darwinist policies.
Social Darwinism essentially denies one of the fundamental tenets of civilization, the idea that people in a society have some obligation to protect each other from harm, and is therefore destructive of the goals of a civilized society.