XP System:

Near the bottom of your screen is an orange bar.  This is the XP bar.  When it fills up completely, you can click an XP skill from the bottom right and use it for a period of time by right-clicking.  The available XP skills are:

Warrior:

Cleave = Deals powerful damage to one unit.  Can be bought in Cronus and learned at level 4.  It is the only skill warriors can learn on their own.

Berserker = Deals great damage to multiple units.  Must be level 30 to use.  Can be bought at ElvenCity.

StarSword = Strongest warrior XP skill.  Does massive damage to multiple units.  Must be level 60 to use.  Can be bought at Dark Marsh.

Mage:

Firebolt = Deals powerful damage to one unit.  Can be bought in Cronus and learned at level 6.  It is the only skill mages can learn on their own.

Aureola = Receive extra experience from monsters who are hit by this spell.  Must be level 7 to use.  Can be bought in Cronus.

Purification = Receive extra experience from monsters that are already dead proportional to their HP when they were alive.  Must be level 7 to use.  Can be bought in Cronus.

Traps = Create traps to increase your abilities.  Must be level 7 to use.  Can be bought in Cronus.

Rage = Greatly damage multiple enemies.  Must be level 30 to use.  Can be bought in ElvenCity.

Thunderbolt = Massively damage multiple enemies, but cannot be used as quickly.  Must be level 30 to use.  Can be bought in ElvenCity.

InfernoFire = Strongest mage XP skill.  Does massive damage to multiple units.  Must be level 60 to use.  Can be bought at Gobi Desert.