Mmk, well there are two main definitions of "good" which I can think of:
(1) Those things which satisfy our desires and goals, and which we believe
should exist, or
should be done. In this sense, "good" and "evil" are simply
expressions of our attitudes, and don't really carry the weight of truth or falsity to them. For example, if I say, "Killing people is evil," this isn't actually a true or false statement, but the equivalent of me saying, "I don't like killing." Thus, it doesn't matter whether people agree or disagree over these things, as they're merely comparing their desires to each other.
(2) Those things which constitute
absolute moral truth. This assumes that there are such moral facts in the universe, which
always should be adhered to no matter what. Those who support this view of good and evil typically believe that these truths come from a supreme, perfect being, though there are obviously other lines of argument (i.e that moral truth consists of whatever the majority of people agree is the truth).
I personally find (2) pretty meaningless, since it assumes an essential knowledge of the facts of the universe which I think humanity lacks (if such facts about the universe even exist, that is). I am very much a fan of the idea that "good" and "bad" are completely undescriptive attitudes.
In response to Coltrane's question, I think a human's goal is defined as the satisfaction of his/her desires, whatever those desires may be. Everybody's going to have a slightly-different, slightly-unique set of principles, so to apply the same set of principles to everybody will never work. Of course, you can usually get away with a small set of principles such as, "everyone should do what they can to ensure the life, health, and freedom of as many people as possible".
I guess that's the up and downside of a philosophy forum: everything is a debate!
Damn straight.