Alignments

A character can be Good, Neutral, or Evil, which doesn't take much explaining. (Really neutral characters are quite rare in the real world, and in many campaigns.) A character can also be Lawful, Neutral, or Chaotic. In general, chaotic characters act spontaneously and don't like laws; lawful characters are rule-followers and probably think things through carefully; neutral characters are in the middle - they either believe in a balance of law and chaos, or they don't care. Each character belongs to one group from each category - Chaotic Good, Lawful Neutral, Neutral Evil, Chaotic Neutral, etc.

Each combination has its own peculiarities, especially when placed in a context in role-play. For example, "Lawful Good" tries to do the right thing and follow rules. "Lawful Evil" uses rules to get a power trip or some dishonest gain - think about an evil dictator or conniving lawyer. What would a Lawful Good character do in a society with a Lawful Evil king and appropriate rules? Leave? Try to not get noticed? Agonize mentally over whether those kinds of laws counted? (Steve McDonald: "Can law be law, when based on wrong?") All of these answers can be right, depending on the character's other personality attributes and about a million situational things - enough different ones that usually, it's left up to player's discretion, because no one can know whether an action is in-character without knowing the character, and no one can know a character better than the player.

A Chaotic Good character in that kind of society would have an easier time reconciling ethical alignment and law/chaos alignment, because a Chaotic can break laws when necessary, bribe guards or form an army with the intention of overthrowing the evil ruler, or - well, just about anything. (Open war might be allowed for a lawful character, but probably wouldn't be the chosen course of action for a character who'd grown up in that society - though in an extreme situation, an honest revolution might be appropriate.)