Collection

A collection is a series of combined artifacts. When a collection is full you can turn them in at a NPC in Meridian/Sanctum for a reward.

Rewards
A Sack of Assorted Rewards such as Planar Essences

Types
In the game interface there are actually five types of collections (listed below), but only the artifact collections are turned in for rewards.
 * Artifacts
 * Mounts
 * Books
 * Titles
 * Companions