Defender Auras are aura 1, so they affect every square next to you. Any enemy adjacent to you is thus "in the aura" and therefore affected by it.
Whenever a creature attacks, it makes an attack roll to try to hit. Some attacks have a single target (such as shooting someone with an arrow), while others have multiple targets (such as a dragon's breath or a wizard's fireball).
Whenever an enemy in your aura (that is, an enemy next to you) uses an attack, and none of the targets of the attack have the Defender Aura ability, the enemy takes a -2 penalty to all attack rolls for that attack.
Example 1: Dan the defender has a defender aura. Mike the monster, Dan's enemy, is 2 squares away, and spits acid at Dan's friend, Wally the wizard. Because Mike is 2 squares away, he is not in Dan's defender aura, so the aura has no effect on Mike's attack.
Example 2: Dan the defender and Wally the wizard are flanking Mike the monster. Mike swipes at Wally with his claws. Because Mike is next to Dan, Mike is inside the defender aura. Mike's attack targets only Wally; because nobody targeted by the attack has a defender aura, and Mike is inside a defender aura, Mike takes a -2 penalty on his claw attack.
Example 3: Dan, Mike & Wally are all adjacent to each other (forming a letter L). Mike breathes fire on Dan & Wally. Mike is inside Dan's defender aura, but one of the targets of his fire breath attack (Dan) has a defender aura. Therefore Mike does not take any penalties on his attack.
Example 4: Dan & Mike are adjacent to each other. Mike decides to spit acid at another of Dan's friends, Doug the defender, who is on the other side of the room. Mike is next to Dan, and in Dan's defender aura, but because Mike's target has a defender aura, Mike does not take any penalties on his attack.
Example 5: Dan & Wally are adjacent to each other. Wally flings a bolt of lightning at Mike, who is on the other side of the room. Wally is inside Dan's defender aura because he's next to Dan, but Wally's attack does not take any penalties from the aura because the aura only affects Dan's enemies.