Handlock is one of the Classic Hearthstone archetypes. Synergies between Warlock’s Hero Power and cards like Twilight Drake and Mountain Giant (as well as Molten Giant at the time) were quickly discovered and abused. The deck wasn’t viable for a long time, especially since Witchwood’s Even Warlock (with Genn Greymane) was simply a better options. However, now that Baku & Genn were forced into Hall of Fame early, Handlock is gaining traction again.
The deck’s basic game plan revolves around Life Tap. You want to abuse a big hand size, making your minions cheaper or more powerful. Depending on the build, you might also utilize “Can’t attack” minions (Ancient Watcher, Arcane Watcher) by Taunting or Silencing them, or you might run cards like Plot Twist that let you reshuffle big hand while potentially giving you more advantages (e.g. heal from Aranasi Broodmother). There is no one “Handlock” list, and for now players are trying a variety of different approaches, but so far some of them look quite promising.