A simple example:

Here is Shaq, an epic level WoW Paladin using magic videogame powers to allow the Rogue DPS to sneak by.
Here is the actual explaination of what is hapening:
What: In basketball, when a player sets a screen to trap a defender, while a teammate whisks by with the ball:
Why: The player with the ball is Phoenix Suns point guard Steve Nash. The player with his back to Nash is his center, Shaquille O'Neal. The player Shaq is setting the screen against is... well, who cares. That guy doesn't matter. Because Nash and O'Neal are using the ol' pick and roll to render his intentions irrelevant. The defender's man is Nash; his coach told him to guard Nash when he had the ball, but the largest mammal on two legs just rotated over from his man to block the defender. Nash now has an uncontested shot. If the defender is fool enough to leave O'Neal, Nash will just pass into Shaq for the easy dunk. Either way, two for the Suns. (If you must know, the defender's name is Marko Jarić, then a forward for the T-Wolves. You will now forget about him.)
Impact: The pick and roll defines why you need both speedy little guys and towering behemoths on a basketball team. A team with a great point guard and one or more immovable objects will score a ton of points. Teams like Phoenix during the Mike D'Antoni era would routinely run the score well past the 100-point mark, daring other teams to keep pace.
As you can see, the NBA has been influenced by World of Warcraft and even uses MMO terminology like marking and defender... or maybe this terminology has existed before MMOs.
There is nothing magical in a Fighter marking someone (unless he is a Swordmage). It's him getting in their face, swishing his sword around and covering his allies. It's like setting a pick in a team sport, you are blocking this guy's path. In D&D you do it with a hurty weapon in hand. In the above example, if Shaq had a longsword in his hand, it would be coming out that defender's back.
This is not the same mechanic as taunting/aggro in videogames, taunting in videogames is what the 3e social skills do. The 4e marking mechanic is taking an opportunity attack against a guy you just whacked who is now stupidly trying to ignore you.
Here is further terminology from a wiki 'bout defender mechanics in futbol:
-Man-to-man marking, or man marking, is a defensive strategy where defenders are assigned a specific opposition player to mark rather than covering an area of the pitch.
-Zonal marking is a defensive strategy where defenders cover an area of the pitch rather than marking a specific opponent. If an opponent moves into the area a defender is covering, the defender marks the opponent. If the opponent leaves this area, then marking the opponent becomes the responsibility of another defender
Ah, Zonal marking! That sounds rather familiar to a D&D4e player (defender aura!)
A picture is worth a thousand words, so here is my 1,000 word essay on 'the mechanics behind a defender aura (ala Knight)':
Ignore the adult bear and attack the bear cub. Guess what is going to happen.[ 942 views ]

