ArcGIS Server REST API is used to access ArcGIS for Server and ArcGIS Online services

The ArcGIS Server REST API, short for Representational State Transfer, provides a simple, open Web interface to services hosted by ArcGIS Server. All resources and operations exposed by the REST API are accessible through a hierarchy of endpoints or Uniform Resource Locators (URLs) for each GIS service published with ArcGIS Server or ArcGIS Online.

For more information see its home page.