Rapid Regeneration: Heals party over time.
Entangle: Deals damage and slows target briefly
Invigor: Heals a large amount of damage and reduces their skill cooldowns briefly
Cyclone: Deals damage to random targets
Healing Circle: Heals party
Renewal: Heals a small amount of damage over time
Wrath of Nature: Entangle and Cyclone deals more damage
Heart of the Wild: Increases defense
Essence of Nature: Increases all healing abilities
Druid is an indispensable hero due to her great healing capabilities as well as her ability to affect skill cooldowns.
The most common skills for Druid are Entangle and Invigor. Entangle is great for slowing down enemies by a significant margin, particularly bosses and problematic heroes in Hero Battles. Invigor does the opposite: it speeds up an ally instead. The best hero to invigor depends on what you're doing, but whichever hero you invigor will become that much (stronger/)faster.
Druid's other skills are more situational than the others. Renewal is a potent healing skill and thus used often, but in some cases it is swapped out in favor of Cyclone for extra damage (leaving the soul skill, Rapid Regenation, to do most of the healing). On the other hand, Healing Circle may be used in place of Entangle to get the most amount of healing possible. As far as passive skills go, Essence of Nature is almost always used, with Wrath of Nature equipped for DPS and Heart of the Wild otherwise.
Druid can use a number of different weapons. Good staffs for pure support purposes include the Abjurer's Staff of Protection, Nature's Offering, and even the uncommon Spirit Staff which provides a great boost in defense comparable to some rare and even super-rare gear. However, one may consider using Rod of Ruin to boost the damage of her Entangle and/or Cyclone skills. Upon reaching level 55, the White Staff of Viarus is the best weapon for Druid, providing superior offensive and defensive capabilities. As for armor, Great Wyrm Hide is by far the most protective armor to use until level 55, in which case Living Leather is favored for offering much more health, armor, and Poison Resistance than its predecessor, supplemented by its bonus regenerative property. Living Leather also benefits from Druid's Essence of Nature passive, granting an extra 43 points (at max) to the regeneration property, totalling to 118 points every 3 turns.
Hide of Sargothian is good for a DPS Druid because of the stacked Wrath of Nature passive, and gives a health boost but only a stun resist. For this reason, it's better to rank her up to improve her defense, or just wait until around level 50 or so when she has high health and defense anyway. Equip Invigor because of the 5% slower cooldowns. Be aware of this in any builds using Hide of Sargothian.