One of the glittering hallmarks of Dungeons and Dragons is the various alignments that you must choose for your character at creation. The classic "Chaotic vs. Lawful" axis no doubt is a tribute to the Melnibonean Mythos (Elric of Melnibone, Stormbringer, Weird of the White Wolf, etc.) by Michael Moorcock, while the quintessential "Good vs. Evil" axis likely pays homage to the Lord of the Rings trilogy (as well as numerous other classic tales).
Alignment is a non-trivial choice. Numerous spells, weapons, and magic items will affect you positively or negatively, more or less extremely, or not at all based on your alignment. Many classes are hard-wired to a particular alignment (Paladin, Monk, Druid, Barbarian, Bard, etc.); clerics certainly feel the effects of their alignment throughout their lifetime.
For the record, I have often been critical of the rather vague and broadly generalized D&D alignments. I'm much more partial to the GURPS way of fleshing out one's character with quirks and disadvantages. I find it a much richer, more intersting, more realistic, and far more amusing way of explaining your character's motivations, limitations, and behaviors.
That having all been said, I've developed an appreciation for the classic D&D alignments as being very well suited to the fantasy genre. I've also come to see them as large categories like "male" and "female" that leave a lot of room for variation. However, most players I've met don't take the time to sculpt out their alignment with some finer edges, leaving it a crude block of granite.
So, with that, dear reader, I present to you some clarifications, synonyms, and examples that you can use to help more clearly define your character in terms of their alignment. Enjoy. Additional suggestions are always welcome.
Debates about what would be in accordance with or a violation of one's alignment are frequent. Semantic arguments are bound to arise when vaugaries exist. To that end, here are some clarifications given to help more clearly define the scope of the basic alignments.
The core of a Lawful character is that he obeys the rules of man: church, state, corporation, etc. and not some "higher" rules, such as the laws of God (more fitting for the "Good" alignment) or the objective dictates of logic (more fitting for the "Neutral" alignment).
Likewise, the essence of a Chaotic character is that he does not follow the rules of man, but blazes his own trail and is considered "a rebel" by society (even if he doesn't think of himself that way). A Chaotic character may even believe that he's following some "higher law", even if nobody else believes that about him.
"Leaders" are somewhat difficult to align because, while their style of rule might be completely raving (i.e "chaotic"), they are the law. Tricky one there.
A Good character is primarily interested in the welfare and happiness of his fellow man, possibly even to the point of putting himself at risk to help others. Ideally, a Good character wants a "win-win" that will benefit himself, his friends, and hopefully society as a whole.
An Evil character on the other hand is so overwhelmingly selfish that he will use, manipulate, decieve, and even kill other people to gain an advantage, accomplish his desires, or get himself out of trouble.
Side Note: As a DM, I always prohibit players from making Evil characters because none of the Evil attributes (see the synonym list below) are desirable in a heroic character; the Evil alignment is reserved for the bad guys. (Plus it makes it that much more interesting when a magically-induced alignment change takes place...)
When Neutral is combined with one of the other alignments (Lawful, Chaotic, Good, Evil) it usually suggests a "pure" form of that alignment. For example, someone who is Neutral Good obeys a higher ethos of right-living that is not beholden to man's laws. Likewise a Lawful Neutral person would be solely concerned with satisfying the rule of law, regardless of whom it helps or harms. In this way, Neutral is rather like an underlying canvas that is totally concealed by the paint of another alignment.
A true neutral character presents some interesting challenges. One of the best clarifications I've heard is to decide whether your character is "Neutral by choice"; the complement being, I imagine, Neutral through lack of experience or consideration. A character who has chosen to be true Neutral is likely well-informed on the matter and has made a concientious decision to stay above the fray, where an inexperienced true-Neutral character is probably just indecisive, flighty, apathetic, or not very thoughtful about it.
Lawful: rules-oriented, orderly, organized, law-abiding, traditional, given to routine, adheres to customs, regimented, hierarchial / top-down, follows the chain-of-command, conformer, easily governed, concerned with legality / legitimacy, submits to / is the authority, makes / adheres to judgements, licit, sanctioned, inspired by a "just cause", judgemental, refereeing, systematic, methodical, "like clockwork"
Chaotic: unpredictable, spontaneous, non-traditional, footloose, answer to no one, make your own rules, helter-skelter, disorderly, disorganized, confusing, muddled, wild, unrestrained, "no boundaries", party animal, out of control, untamed, deviant, exuberant, crazy, delerious, delusional, raving, uncivilized, undomesticated, lawless, scrambled, thrown together, slipshod, makeshift, "fly by the seat of your pants", rebelious, non-conformer, maverick
Good: kind, helpful, caring, compassionate, loving, brave, courageous, moral, positive, optomistic, pleasant, admirable, honorable, respectable, benevolent, altruistic, honest, truthful, virtuous, principled, beneficial, saintly, gracious, worthy, angelic, righteous, friendly, selfless
Evil: selfish, murderous, treacherous, tyranical, sneaky, backstabber, unkind, lying, decietful, hateful, derisive, abusive, corrupt, wicked, harmful, destructive, immoral, depraved, vicious, injurious, malevolent, malfeasent, malign, nefarious, cruel, insensitive, atrocious, heinous, monstrous, brutal, bad, sinister, dark, hostile, perverse, demonic, diabolical, feindish, hellish, infernal, despicable, vile, unworthy, reprehensible, devilish, unrighteous, theiving, nasty
Neutral: undecided, objective, indecisive, no-opinion, apathetic, even-keeled, temperate, fence-sitter, not taking sides, impersonal, observer, no preference, indifferent, inactive, non-descript, lacking distinction, makes no impression, amoral, unbiased, nonaligned (in the "allegiance" sense of the word), unemotional, doesn't react, passive, viewless, balanced, logical, impartial
I've freely mingled real, historical figures with literary characters (as I am wont to do). "Generic" personas are listed in double quotes.
The Tick ("Lawful Stupid")
Sidhartha Gautama (the Buddha)
Bilbo & Frodo Baggins
The Dali Lama
Ben (O-Bi-Wan) Kenobi
Fred & George Weasly
Kang from "Kung Fu"
"Cop just doin' his job"
Peeves the Poltergeist
Hitler & the Nazis
Mao Tze Dong
"Cop on the Take"
Long John Silver
Jack The Ripper
Baron Vladamir Harkonnen
Blackbeard the Pirate
Khan Noonian Singh
Online Alignment Test on Wizards.com. Has a 36 question test you can fill out to determine what your alignment would be. Do it for yourself or for your character.