Experience, often abbreviated XP, is gained from completing quests, killing NPCs, crafting and gathering. This experience go towards a characters overall level, with each level having a required amount of experience to level up.
Quest experience is by far the most lucrative: experience gained from quests is normally vastly more than the experience gained from killing NPCs in the same time scale. Experience from quests are given at checkpoints throughout the mission, with some quests being expecting a collection of goods, or a culling of enemies.
NPC experience can be gained from any part of Atreia. As long as an NPC can be slain, it will hold experience, unless their level is too low for the character's level. Experience gained from an NPC kill is based on the level of the NPC - as a rule, the higher the level of the NPC, the more experience gained. There is a limit, however, as if an NPC's level is grey when moused over, this means that very little experience will be gained from a kill.
Crafting experience is gained from successfully completing an item or work order. Higher level items give more experience, but require more time and resources.
Throughout Atreia, characters are able to Collect or Essencetap certain items. These items branch from minerals like Crystal, metal like Iron, or plants and herbs like Hasia. With a successful collection of a resource, the character gains experience. As with all experience, the higher the level requirement to collect, the more experience given.