Druids are specialist healers, with their secondary defining how they heal.
Druids cannot have Knight or Priest as their secondary.
D/M: The Druid/Mage actually sports DPS rather than healing. A hybrid healer/dps combo, this one is neither excelling in either, but has a balanced mixed of dps and healing.
D/R: The Druid/Rogue leans more towards DPS, focusing on Dark damage, crit, DoT’s, and powerful single-strikes.
D/S: The D/S is generally termed as the ~best~ druid healer, focusing on pure healing. Specialists in single-target, these healers can ake any class a tank.
D/Wd: Probably next in line for Druid healing, thrown in with some damage skills.
D/W: A really wierd healing combo, with really, depending on how you stat, can be a healer or DPS. DPS is similar to the P/W, healing is more like a mobile Group HEal.
