There are 2 different arenas you can join to mix things up from time to time. You can find them both on the outskirts of Flotsam Tower.
The PVP arena is a barebones drop in and PVP arena. There are no rewards except the pride you get from slicing your peers into bits.
The mob arena is a featureful wave-based arena with classes, unique currency, upgrades and a rewards shop. You can play in the arena by yourself but its much funner with friends so invite someone to join you if its empty.
The 5 classes are: Druid, Hunter, Knight, Paladin & Rogue
Every mob you kill will reward you with raw experience.
You can spend you earned nuggets on class upgrades.
You can also spend your nuggets in a special rewards shop.