Elite enemies
Elite enemies are special monster variants that are significantly harder to deal with. They boast increased health pools and unique offensive capabilities. An elite enemy is recognized by their increased size, a particle effect and an icon overhead that indicates their type.
Any non-spawned monster can be elite, even red crystals (that don't have attacks of their own) and poppers (that destroy themselves on attack). Once destroyed, elite enemies drop a pile of treasure, a yellow life orb and an uncommon weapon as a reward.
Elite enemy spawning rate can be adjusted using pills.
Elite types
As of v1.0, there are six types of elite enemies:
Elemental
Fire elites set player on fire for as long as player is in vicinity. In addition to that, their attacks are explosive and imbued with fire effect.
Ice elites slow player down while nearby. Their attacks also carry ice effect.
Shock elites electrify player with their presence, making it impossible to use weapons. Their attacks are also augmented with electricity.
All vicinity elemental effects apply through walls, making it possible to surmise that an elite enemy is nearby before it is encountered.
Other
Bomb effect turns any monster into a "walking barrel" of sorts, exploding violently on death. It's possible to use this in player's favor by dealing with them at safe distance, while they're surrounded by other monsters. However, their movement speed is also increased, making it easier for them to approach player. This makes fast melee enemies like Drowners especially dangerous.
Ninja elites have a high chance of dealing critical damage to player. If left unchecked, they drain player's health pool faster than the player realizes what's happening.
Shield elites are extremely tanky. They have so much health that they drop ammo when hurt to compensate for this.
Trivia
Shield elites' health multiplier has undergone several nerfs both during beta testing and after release.