The Druid class are one of the most versatile classes to play in World of Warcraft and can fill four different class roles which are; Tank, Healer, Ranged Magic Damage Dealer and Melee Damage Dealer. A Druid works closely with nature and can unleash the power of nature upon their opponents on top of this the Druid possesses the ability to "shape-shift", where they can take on the form of a bear, cat, aquatic seal, cheetah, moonkin or tree.
The Druids ability to shape-shift gives them a nice option for different gaming scenarios. For instance when they're in bear form they are more resilient to attacks and have increased defense, when the Druid assumes the form of the cat they become more close range melee damage dealers savagely clawing their opponents to death!
There are currently only available races that can play a Druid and they are the Night Elf and Worgens on the Alliance side and the Tauren and Trolls on the Horde side. One shouldn't find this to restricting though as there are a number of different talent builds available to the Druid to meet just about any gaming scenario.
The three talent trees available to the Druid class are the Balance, Feral and Restoration talent trees. Each of the three talent trees focuses on one or two of the Druids shape-shifting abilities and each offers a different build type whether it be healer, tank or damage dealer.
The Balance talent tree will focus on the Druids Moonkin form where Arcane and Nature magic are combined to annihilate opponents from a distance. The Feral talent tree will focus on both the Druids Bear and Cat forms where damage output in melee is increased in Cat form and damage taken is reduced in bear form. The Restoration tree focuses on a Druids healing capabilities as well as tanking and offers many abilities that are beneficial to both the Druid and group members.
If you're looking to level the Druid class quickly as an "alt" in solo play then the Feral talent tree and Balance talent tree are the best options. I would opt for the Feral build of the two as this will improve the Druids cat form and bear form meaning you'll have a nice Druid leveling build for wiping out the mobs and reserving health while solo questing.
The restoration tree should be used by players who're looking to get really involved in group activities such as dungeons and raids. There are lots of abilities that will provide nice buffs for both the Druid class and that of their allies to be found in the Restoration tree. This makes the Restoration Druid a welcome member and guest among groups and parties.