Sarcasm is the art of saying exactly what you mean, while apparently saying the opposite.
Up until the rise of the Roman Empire, sarcasm was purely a military tactic. Battalions of Sarcoids would wander nonchalantly across the battlefield toward the enemy, swords hidden beneath their thingies, loudly but casually saying things like, "Ain't no way I'm going to fight today, I'm far too hungover — besides, who the fuck would want to fight a bunch of women and children, then rape the survivors and loot their absent husbands' treasures? Not I, that's for sure!" Then, when they had lulled the enemy into letting its guard down, they'd suddenly rush forward and stab them all in the throat, while triumphantly bellowing, "Ha! Gotcha!"
Later on, the Vikings tried the same thing, but the naturally sarcastic British saw through the ruse immediately, and ran away.
When a politician wishes to address another politician, no matter how much he hates him and wishes to torture him to death using iron spikes and squashing-machinery, he will address him as "My Right Honourable colleague." This is official, government-sanctioned sarcasm at its finest. "Finest," of course, itself being sarcasm, which means, "This is the sort of dopey nonsense we pay these parasites a bloody fortune to utter, while they shaft us."
An example of political sarcasm is: "Is my Right Honourable colleague aware that since he first took office, a miraculous concept has been invented, which those of us 'in the know' call 'democracy'?" Following such a sentence, the Sarcoid will turn to his real colleagues and smirk, while rocking on his heels and sticking out his chins, to (he believes) universal approval.
On the street
When you meet an anorexic or a bulimic, do you find yourself wanting to call them 'Fatty'? This is an example of common sarcasm, the type practiced by children and immature adults every day, in every country on Earth. Being vulgar is second nature to the vast majority of humans, and sarcasm is a primitive and deeply misguided attempt to sound clever, witty and sophisticated. Small children learn to use sarcasm from their parents at a very young age, for instance, "Oh, you are SO funny", meaning "You are so NOT funny". This paradox is the essence of sarcasm, and explaining it to an alien would be far from easy. You can imagine the alien nodding slowly, and tentatively saying "Do NOT take me to your leader, HAHAHAHA!" and looking all crestfallen when the government spokespersons 'don't get it'.
Uncyclopedia is a glowing example of the unsuccessful deployment of sarcasm, and not all of it by aliens. Rifle through the pages and you will find such gems of wit as "George Bush is SO the gratest prssident the USA have ever not known, he suks big time and cin go kiss my skiny whit ass!!!" (Brad Pitt on US politics). Indeed, Uncyclopedia may be the greatest repository of failed sarcasm in the world. Mr Pitt's regular and enthusiastic contributions to Uncyclopedia, allegedly proofread by his wife, form a remarkable testament to the eternal optimism so contagious among Americans: the belief that everything they say, no matter how banal, facile, crude or stupid, is a rare nugget of insight and humour.
In international affairs
Americans export almost every new fashion trend to Europe using television to bombard 'the old country'. Programs such as The Simpsons, Friends and Seinfeld are an unending torrent of pure, pseudo-sophisticated sarcasm.
Even so, sarcasm was not invented by the USA but by Europeans, who called it irony. Irony differs from sarcasm only slightly, and the difference is bitterness. If you are marooned on a desert island, and six years later a cruise ship full of nymphomaniacs passes close by, irony would be George Clooney choosing that moment to pass by in his steam yacht "Phallic Symbol II" on the opposite side of the ship to your island thus causing all the nymphomaniacs to rush to the rail to see him. A double irony would be if they caused the ship to capsize, but instead of swimming to your island, drowned because they were a delegation of non-swimming nymphomaniacs. Sarcasm would be what you, on the island, would later that day deploy, and it would go something like: "Well that was a lucky break, we're fresh out of condoms".
For sheer smugness and the ability to deeply irritate, the French beat the Americans every time. Restaurants in Paris actually refuse to serve Coca Cola, and a request for it will be met with a sneer and a comment such as, "Perhaps sir would feel more comfortable at a different establishment — for example McDonald's? I believe sir will find one on the Champs Elysees. Would sir like me to call him a taxi?"
In Germany, sarcasm is delivered with all the subtlety of Luciano Pavarotti's dead corpse being dropped from a Zepellin. "So, Uncle Fritz- what did you do during the War?" "Vell, I vos mindink mein own business, you know, ven all of a suddenly, "Pooof" ve are attacked by ze Britishers and ze Americaners at simultaneous times! Vot choice did ve haff but to occupy Poland! Und I shot down three of zose Fokkers. Zey were flying Messerchmitts."
Russia has its own variation on a theme, but Russian sarcasm is bound up with a depressed national psyche and is of the darkest possible variety. You are climbing the stairs to visit your favourite prostitute and you pass a bearded, stoop-shouldered ancient old man, clad in black. "Good morning Comrade!" you ejaculate. The weary old man pauses for a millisecond, darts a suspicious glance at you from under his bushy eyebrows, spits professionally into a rusting bucket he carries in one mittened hand, and growls, "Good! Ha, yes I suppose you good say it is a good morning- last night my wife died coughing up blood all over my favourite rug." Um, what was good about that, old man?" you retort flaccidly. He spits again, and fixes you with his bloodshot eye, "It needed cleaning anyway. Now, out of my way, before I eat you."