Characters start at level 1. Cap is 100. Experience to the next level:
300 + (120 × current level)
Mobs also grant 0.5 XP per point of their health.
On death you lose 20% of current progress to the next level, not all stored XP. If a player kills you, they receive the lost XP; assists get 20% of that reward.
Alchemist passively gains +30% personal XP. Intelligence also speeds XP gain; on Alchemist that Intelligence bonus can be strengthened by the Gold Rush talent.