Identity service (keystone) provides identity, token, catalog, and policy services for use specifically by services in the OpenStack family. Identity service is organized as a group of internal services exposed on one or many endpoints. Many of these services are used in a combined fashion by the front end. For example, an authentication call validates user and project credentials with the identity service. If successful, it will create and return a token with the token service. More information can be found by reading the keystone Developer Documentation.