Each zone in the game has 1 lair and at least 1 farming area.  Both types of areas have mobs with special buffs, often giving them immunity to certain debuffs while placing debuffs on their attackers.  Lair mobs will have a "Lair" buff, while farming area mobs will have a "Nightmare", "Triggers" or similar buff.

Mobs in both types of areas have a chance of dropping gear, currency, runes, toolkits, sequins, and signets.  The main difference is that lair mobs have a greater chance to drop major/head/weapon signet, and they also drop blueprint pieces used to summon lair bosses and have a chance to drop a Bind on Pickup rare core item. Each lair also has a quest available at the site of the lair.