This is mainly a tanking minion, and is not the best for damage. Earth elementals have a fair amount of health, roughly as much as the shaman does; as such, while they excel at handling mobs out in the world, elite monsters will kill them rapidly. Earth elementals deal physical damage, but as with any earth elemental, they are immune to all Nature spells and effects.
Casting another [Earth Totem] will destroy the elemental. Elementals have a fairly large aggro range and may cause problems in instances by attacking mobs that were not otherwise in combat, much like [Searing Totem], although the elemental is not aggro-linked to the shaman, and when it dies the mobs will reset as long as no-one else is on their threat list. Elementals move surprisingly quickly, and use an AoE taunt ability frequently.
In instances, the primary use for Earth Elementals is to serve as an emergency magnet. Their AoE taunt will almost always gain the attention of any mob that is affected by it - while monsters focus their attacks on the elemental, they are gathered up for another player, preferably the tank, to deal with. As such, they serve as a bit of an "Oh no!" totem. The Earth Elemental can also act as an emergency tank in and of itself if the tank dies, especially if the shaman that cast it is a healer and can keep the elemental alive, preventing a wipe.
Tips and tactics
The Earth Elemental is a great soloing companion for harder (group) quests. Certain quests that normally would require a 3 or even 4 man group can easily be soloed with an Earth Elemental Totem and a competent Shaman. A good tactic involves constantly switching between healing the Elemental, healing yourself, and dealing damage.
Patch 6.0.2(2014-10-14): Deals 90% less Auto Attack damage but generates 1000% more threat.
Patch 5.4.0(2013-09-10): Summoned Greater Earth Elemental will no longer taunt off players that are in a tanking specialization.
Patch 5.0.4(2012-08-28): Duration reduced from 2 to 1 min. Cooldown reduced from 10 to 5 min.
Patch 4.0.6(2011-02-08): The shared 2 minute elemental cooldown has been removed. Earth and Fire Elementals can now both be used at once. (undocumented)
Patch 4.0.1(2010-10-12): Shaman are no longer required to carry around totems in their bags.
Patch 3.3.0(2009-12-08): The cooldown for this totem has been reduced from 20 minutes to 10 minutes. Cannot be used in Arenas.
Patch 3.1.0(2009-04-14): The summoned Earth Elemental should now have significantly more health and slightly more armor. Now scales up in power based on the shaman's spell power instead of attack power.
Patch 2.1.0(2007-05-22): This pet will now come into the game with full health, including that gained from a percentage of its master's stamina.