CRM 2015 Hierarchies are a really cool feature that let users navigate records by visually seeing parent child relationships. Out of the box Accounts, Positions, Products, and Users entities get hierarchies baked in. Hierarchies can be added to other built-in entities and to all custom entities.
To make working with Hierarchies easier there are also two new Advanced find operators:
Under: Finds all records that are UNDER a specific node
Not Under: Opposite of Under