Alignment in D&D is one of those perennial (and perennially frustrating) topics of discussion, because everyone has their own ideas about what the words mean, and the definitions provided in the books are overly vague and generalized to the point of being essentially meaningless, so there's a lot to argue about and for people to accuse each other of being both Wrong and possibly A Bad Person.
What it ultimately comes down to is that each individual DM should decide how they want to define and handle alignment in their campaign world and let the players in that campaign know what they've decided early on rather than assuming everyone is in agreement and then getting into arguments later on when the players do something based on a different interpretation than the DM. While that's surely wishful thinking (because players who disagree with how the DM defines things aren't going to disagree any less if they see it in writing in advance) I've nonetheless collected some notes both on how I view the alignments in my games along with some representative examples of characters from fiction and media who I feel fit into each of the alignment buckets, and figured since I've gone to the trouble to write this stuff down I might as well share it. The examples serve a second purpose as well, by drawing characters from a variety of sources that I'm interested in and feel are relevant to the style and flavor of the games I like to run (as opposed to other lists you can find online, which tend to be limited solely to examples from comics, Star Wars, and Harry Potter).
AD&D ALIGNMENT NOTES
Lawful Respects (and expects) authority and loyalty derived from formal structures (title, office) and hierarchical organizations
Chaotic Respects (and expects) authority and loyalty based on individual personal qualities (strength, charisma, renown, family ties) and fluid or informal power-structures
Good Seeks to help others, especially the weak
Evil Seeks to exploit others, especially the weak
Neutral Pragmatic, opportunistic, or indifferent
Representative examples from fiction and media:
Lawful Good John Carter (Edgar Rice Burroughs), Agent Dale Cooper (Twin Peaks), Roland Deschain (Dark Tower), Hermione Granger (Harry Potter), Spock (Star Trek)
Neutral Good Cazaril (Curse of Chalion), Jake Chambers (Dark Tower), James T. Kirk (Star Trek), Harry Potter, Shimrod (Lyonesse)
Chaotic Good Lyra Belacqua (His Dark Materials), Eddie Dean (Dark Tower), Katniss Everdeen (Hunger Games), Kickaha/Paul Janus Finnegan (World of Tiers), Peter Pan
Lawful Neutral Judge Dredd, Inspector Javert (Les Misérables), Agent Albert Rosenfield (Twin Peaks), Severian (Book of the New Sun)
True Neutral Ged/Sparrowhawk (Earthsea), The Gray Mouser (Fritz Leiber), The Man With No Name (Sergio Leone movies), Nifft the Lean (Michael Shea), Rhialto the Marvelous (Jack Vance)
Chaotic Neutral Harry Mudd (Star Trek), Loki (Marvel movies), Skafloc (The Broken Sword), Captain Jack Sparrow (Pirates of the Caribbean)
Lawful Evil Casmir (Lyonesse), Lady/Dorotea Senjak (The Black Company), President Snow (The Hunger Games)
Neutral Evil Kane (Karl Edward Wagner), Khan Noonien Singh (Star Trek), Steerpike (Gormenghast)
Chaotic Evil BOB (Twin Peaks), Cugel the Clever (Jack Vance), Voldemort (Harry Potter)
