Self relationship in Salesforce

A Self-Relationship is a lookup relationship to the same object. For example : we have a field on Account which is Parent Account that allows us to choose any account for being a parent account of an existing one.

Self-relationship:
When an object has a lookup with itself, it is a self-relationship. A self relationship creates a tree diagram of the objects. For example, the account has a lookup on himself, called Parent Account.
On the user object a special self-relationship called the hierarchical relationship helps create superior roles such as supervisor, manager on user object. The hierarchical relationship is a self-relationship of the user object.