I have alignment descriptions in my House Rules document because new players of "good" characters started doing very evil things. It's long, so I'll put it in spoiler tags.
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Good Thing to Know: Alignment
Characters are expected to play their alignments. This isn't a straight jacket; there is a lot of room within an alignment if you are creative. Alignment is in place simply to keep an idea about your character's general attitudes. Acts outside your alignment won't change it immediately, but repeated acts will cause it to shift. This can be an overnight process or a process taking several in game years. A slow changed can be interesting character development. Here is a brief breakup of the alignments:
Moral
Good: Good characters help others even when there is no direct benefit to them. They are willing to sacrifice personal gain in order to help others, and it is this extent which separates them from Neutral characters. Good characters will help those they don't know and often those they don't like because they feel it is what they should be doing. This doesn't mean they won't kill, it means that they are willing to aid those who need it.
Good characters don't necessarily go out and smite evil whenever they can. They can have characteristics such as greed and jealously which are normally evil characteristics; they just have more control over these impulses and wish to reign in these more negative thoughts. Good doesn't mean stupid, they can be calculating and devious, but not to the point of hurting others for pure personal gain. Above all they wish to help make the world a better place to live for everyone.
Evil: Evil characters want to help themselves. They might not necessarily want to injure others, but they will not hesitate to step over others to gain something themselves. Where Good characters are defined by making sacrifices to aid others, Evil characters are defined by using others in order to aid themselves. Looking out for Number One is the Evil character's creed and devotion.
Evil characters don't necessarily go out and kill for fun. There is often a method to what they do, and many Evil characters do have their limits. Not every Evil character will be a murderer; they may just be cheats and con artists. Evil characters can be generous and kind to those of their inner circle, but it is important to note that if those of the inner circle ever steps out of bounds or lose favor, then they could be turned on very quickly by an Evil character with little or no hesitation.
Neutral: Neutral alignment on the Good/Evil axis is an often mistaken alignment. It doesn't mean good and evil acts are permissible as normal day to day events, doing good one day and doing evil the next to balance it out. "Balance" of the 2nd Edition Advanced Dungeons and Dragons is a dead concept. Neutrality means that you will help others, but not at risk to yourself. You'll help yourself to what you want, but you won't hurt others to attain it. A Neutral character usually is single minded in a particular endeavor so as not to be too worried about saving others or killing someone to get what they want. They are more likely to work with Good characters because Evil characters' hurting others is distasteful to them.
This isn't to say that they are incapable of extreme action. Morally Neutral characters will often help those that they find to be friends, making personal sacrifices for them as a Good character will make for others, but these cases are usually restricted to those that they know personally. Neutral characters will not murder or kill for profit and they will not turn on their friends, as those are both evil acts, a Neutral character will do what they feel is right, so long as it isn't too dangerous or involving for them.
Ethical
Chaotic: Chaotic characters hate control and dominance. They don't like societal constraints, and they don't like the idea of someone telling them what to do. They act on their own by their own beliefs and allow others to do as they please as well so long as it doesn't interfere with their own actions or desires or go against their morality. Freedom is the most important thing when Chaotic alignment is involved.
Chaotic characters can have an honor code and a sense of right and wrong of course. This isn't an excuse to do whatever you feel you want to at the time, and it isn't an excuse to change character motivations every day. Chaotic character can have goals, hopes, and dogmas just as much as any other character, its just that these ideas will have to do more with freedom than rigorous constraints on themselves and others, even though they can have constraints upon themselves.
Lawful: Lawful characters like to have order in their lives. They have rules that they follow in order to make their life run smoother and more efficient. Without rules things would just fall apart. They may or may not believe in the laws of a kingdom or such, but they will respect them and understand the necessity for their existence. A Lawful character will have some kind of code or rules to live by and will rarely break these rules because society is built upon such rules.
This isn't to say that Lawful characters are always completely predictable and boring. A Lawful character doesn't regulate every portion of their lives, and many characters are Lawful more for their organization, belonging to a group, and love for tactics, while normal in a day to day setting. Lawful characters do tend to want to know lots of details, however, and they like to form a clear picture of what is ahead. They are also much more likely to keep a deal than a Chaotic character, believing that agreements are important.
Neutral: Neutral characters have no pervasive feelings over Law and Chaos. They do what they want, but usually within some constraints. A Neutral character will usually follow the rules more out of a sense of ease than of any feelings of necessity, and they will break the rules, although without fervor, if they think they need to. Their day to day lives don't concentrate on their own personal discovery or a strict following of schedule. A Neutral character will neither stand up for individuals as a Chaotic character would, nor stand up for an organization, as a Lawful character would, instead being more swayed on moral grounds or personal grounds than overriding beliefs. Often, they'll just ignore such struggles, however, as unimportant.
It is possible for an ethically Neutral character to fight for a certain organization or help freedom fighters gain independence. It is just that they won't tend to do so because of ethical beliefs, which they don't feel strongly about being Neutral. Like morally Neutral characters, they are usually single minded in their pursuit of a goal or in this case possibly a moral pursuit.
Putting it all Together
Lawful Good: Help others whenever possible. Society is important and the needs of the many outweigh the needs of the few (or the one).
Chaotic Good: Help others whenever possible. Aiding the individual is more important than helping organizations.
Neutral Good: Help others whenever possible. Making the world a better place for everyone is the most important thing.
Lawful Neutral: Rules are a way of life and those who don't believe in the rules are dangerous to society.
Neutral: Nothing is really all that important except your personal goals. Help others when you can and don't hurt others, but that's secondary.
Chaotic Neutral: The world is waiting to be explored. Don't do anything to sacrifice your freedoms.
Neutral Evil: Everyone is a stepping stone on your way to the top. You are the most important person out there.
Chaotic Evil: Do what you want when you want, and don't worry about collateral damage.
Lawful Evil: The world is there for you to control and manipulate. Others are merely tools within your machinations.
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Critique is welcome. This is specifically written for people who have no previous knowlege of alignment.
EDIT: And specifically for people who have never actually roleplayed before.