A quick beam of solar light burns the enemy for (20% of Spell power) Nature damage and then an additional (104.4% of Spell power) Nature damage over 12 sec
Rank 2 (Level 32): to the primary target and all enemies within 8 yards
Balance Generates 2 Astral Power.
Moonfire Rank 3 (Level 33) Moonfire and Sunfire duration increased by 6 sec.
Burns the enemy for (20% of Spell power) Nature damage and then an additional (104.4% of Spell power) Nature damage over 12 sec to the primary target and all enemies within 8 yards.
It's also good for kiting in PvP or finishing off fleeing enemies.
It's often considered an excellent choice for pulling, regardless of the druid's specialization due to its instant cast and long lasting DoT.
Trivia
Sunfire is a rare example of an ability that appeared in the Trading Card Game before it actually appeared in the game.
Patches and hotfixes
Patch 7.1.0 (2016-10-25): Now learned at level 16 (was 24).
Hotfix (2016-09-23): "Sunfire damage increased by 10% for Balance."
Hotfix (2016-08-22): "Moonfire and Sunfire DoT damage has been decreased to 50% per tick (was 55%)."
Patch 7.0.3 (2016-07-19): No longer available to Feral or Guardian. Direct damage increased by 157%, damage per tick increased by 96%. Now lasts for 12 seconds, down from 24 seconds.
Patch 6.2.3 (2015-11-17): Damage has been increased by 6%.
Patch 6.2.0 (2015-06-23): Now deals 4% less damage.
Sunfire now has a base duration of 24 seconds (up from 14 seconds), but is no longer extended by [Wrath] and [Starsurge] critical strikes. Enemies may still suffer from both Moonfire and Sunfire’s periodic damage components simultaneously. Upon reaching 100 Solar Energy, the next Sunfire deals 100% additional initial damage.
Patch 7.1.0 (2016-10-25): Now learned at level 16 (was 24). 
Patch 6.2.3 (2015-11-17): Damage has been increased by 6%. 
[Wrath] and 
[Starsurge] critical strikes. Enemies may still suffer from both Moonfire and Sunfire’s periodic damage components simultaneously. Upon reaching 100 Solar Energy, the next Sunfire deals 100% additional initial damage.
[Moonfire] and Sunfire will now replace each other on the Action Bar when switching cycles on the Eclipse Bar. 
Patch 5.0.4 (2012-08-28): Redesigned and no longer requires a talent. 
Patch 4.0.6 (2011-02-08): Now costs 9% of base mana, down from 18% of base mana. 
