In groups, Druids are commonly used as primary healers and can be a suitable replacement for Clerics. They are more often cast as spot healers. Spot healers heal party members other than the tank, or even the tank if the Cleric dies or runs out of mana. They have some damaging abilities, but these pale in comparison to the more DPS oriented classes.
Soloing is a very common and easy way to level for Druids. An easy way for beginner Druids is the Root and Rot technique. This style involves the Druid rooting an enemy to the ground and loading them up on every DoT spell the Druid has available, and then simply waiting for the enemy to die. The Druid will sit and regenerate mana while waiting, and if any of the spells durations expire before the enemy dies, they simply recast and wait.
For those with a more active play style, kiting is a valid soloing option, and once adept at that, quad kiting becomes available. Kiting, and quad kiting involve running while the enemy or enemies trail behind the Druid, while the DoT spells of the Druid tick away at he enemies health. The Snare and Spirit of Wolf spells slow the enemy and enhance the Druids movement speed, allowing the Druid the ability to get distance between them and cast their DoTs before the enemy can get close enough to attack. Quad kiting is the same, except with four enemies rather than one, and should only be attempted by the most skilled Druids.