Dialogue itself is supposedly formed from the two words "dia" and "logos" meaning "to speak across" or to converse.
The best dialogues have a number of characteristics that make them stand out - a distinctive voice reflecting character; a purpose or direction for the conversation; a conversation burned down to the core essentials and shorn of many of the qualifiers, honorifics and interruptions that make up normal speech; and a cadence or rhythm that provides a beat and drama to the speech.
Usually you want your dialogue to reflect the story of the moment, not just the content of what is being said.
Here's some examples stolen from both literature, television and film that leverages some of the above elements effectively:
From The Adventures of Sherlock Holmes, The Man with the Twisted Lip
"I suppose, Watson," said he, "that you imagine that I have added opium-smoking to cocaine injections, and all the other little weaknesses on which you have favoured me with your medical views."
"I was certainly surprised to find you there."
"But not more so than I to find you."
"I came to find a friend."
"And I to find an enemy."
TROMBLEY: Hey, Person. Didn't your mom put your picture up on the Wal-Mart wall of heroes?
PERSON: Yep. My grandma did when I went to Afghanistan. I'm on the Nevada, Missouri Wal-Mart wall of heroes. Even got my dress blues on.
COLBERT: If my mother ever distributed my likeness without written authorization, I would disown her.
PERSON: Technically speaking, Brad, but didn't your biological parents disown you when they put you up for adoption?
COLBERT: Point, Ray. I was one of those unfortunates adopted by upper-middle-class professionals and nurtured in an environment of learning, art, and a socio-religious culture steeped in more than two thousand years of Talmudic tradition. Not everyone is lucky enough to have been raised in a Whiskey Tango trailer park by a bowlegged female whose sole qualification for motherhood is a womb that happened to catch a sperm of a passing truck driver.
Colbert gets out of the Humvee with some humrats.
PERSON: At least my mom took me to NASCAR.
From Pulp Fiction, Quentin Tarantino's classic film:
Yolanda: This place? A coffee shop?
Ringo: What's wrong with that? Nobody ever robs restaurants. Why not? Bars, liquor stores, gas stations; you get your head blown off sticking up one of them. Restaurants, on the other hand, you catch with their pants down. They're not expecting to get robbed. Not as expectant, anyway.
Yolanda: I bet you could cut down on the hero factor in a place like this.
Ringo: Correct. Just like banks, these places are insured. Manager? He don't give a fuck. He's just trying to get you out the door before you start plugging the diners. Waitresses? Fucking forget it. No way are they taking a bullet for the register. Busboy, some wetback getting paid a dollar fifty an hour, really give a fuck you're stealing from the owner? Customers are sitting there with food in their mouths; they don't know what's going on. One minute they're having a Denver omelette; the next minute, someone's sticking a gun in their face.From Lawrence of Arabia:
Bentley: It's very simple, sir. I'm looking for a hero...certain influential men back home believe that the time has come for America to lend her weight to the patriotic struggle against Germany, uh, and Turkey. Now I've been sent to find material which will show our people that this war is, uh...
Bentley: Oh, hardly that, sir. But to show them its more adventurous aspects.
Feisal: And you are looking for a figure who will draw your country towards war.
Bentley: All right. Yes.
Feisal: Lawrence is your man.
Got any favorite dialogue from books or film you want to share?