The name of the tree is a telling name. Warlocks specialized in Destruction use spells that deal direct damage. This makes their playstyle very similar to that of a mage, although it was changed from the simple Shadowbolt spam it was in Burning Crusade. Destruction Warlocks are the only Warlocks who don't use Shadowbolt as their primary direct damage spell. The talents in the Destruction talent tree favor fire damage so Incinerate is the first choice of a Destruction Warlock. At the same time the Warlock will cast a rotation of Immolate and Conflagrate while keeping Corruption and/or Curse of Agony active on the target. This makes for a very complex rotation that gets even more complicated if chaos bolt is used, the ultimate talent in the Destruction tree.
While a Destruction Warlock used to sacrifice his companion at the start of a combat in Burning Crusade this is no longer the case: the Imp gets lots of improvements from the talents, making him a respectable source of damage while having enough mana to last the entire bossfight. Sacrificing your companion is only useful in fights were lots of aoe damage might endanger its survival.

The huge haste bonus it gets from talents, equipment and buffs allow another spell to return into the Warlock's rotation: Soulfire can be used in extreme situations effectively. Unlike the Affliction Warlock the Destruction Lock depends on healers as he is often forced to use Life Tap to keep up his mana and he has no noteworthy selfhealing.