Elementalism Classes
In Elementalism, you start off as an Elementalist, but you are not limited by it. You can choose to change your Class and become a Ninja, Fighter, or Warrior, although the Elementalist Class offers a bit more than all other Classes.
Fire Elementalist has the Combustion and Lightning Subclass.
Water Elementalist has the Plants, Healing, and Blood Manipulation Subclasses.
Earth Elementalist has the Sand, Metal, and Lava Subclasses.
Air Elementalist has the Sound Subclass.
How To Change Class and Sub-Class
Every Subclass gives new unique skills. You can only have 1 Subclass.
When you enter the game, you are an Elementalist by default. If you want to change this, talk to a Class-Changer NPC.
To find them, open Settings (Press N, then click the cogwheel), enable NPC Indicators, and make sure it is set to "ClassChangers".
How To Get The Subclass
Similar to above, open the settings and choose the Subclass trainer. You will need to have certain mastery and in-game currency to learn your specific Subclass.