The available classes are warrior, survivor and mage, though any class can learn any skill so you can build the character however you wish. There is nothing really similar to Diablo's Druid class, though.
differences between classes (minor spoilers)
Early Starter Tips (NON-Spoiler Version) (contains much of the information in the above topic).
The Locus Inn - Skill System (individual paths, and a skill table for each class)
