The Earth Shield heal the user when they are struck every few seconds
When a Restoration Shaman hits the seventh tier, they gain the Earth Shield, which will give ten charges that can be cast Cabal Alz on either themselves or a party member.
The Earth Shield will heal the user when they are struck once every few seconds. Since the Restoration spec will greatly increase a Shaman's contribution to any group or PvE action, it makes it a great way to go for most Shamans.
But, the down side is that this spec will decrease and limit the Shaman's damage to mobs and enemy player.
Further up the Enhancement talent tree, the Shaman can begin to do a huge amount of melee damage and can even surpass some Warriors and Rogues.
But, they will start to lack the ability to reduce threats, so this can make it hard to ward off aggro.
This WoW guide will tell you not only how to power-level your warlock, but will also give you talent specifications so you won't be left in the dark wondering how to power level your warlock.
I found this guide on the main World of Warcraft community website. It is an extremely helpful guide.