The vCenter Server Appliance is an instance of vCenter Server running in a Photon OS™ guest operating system.
vCenter Server is a collection of services designed for managing and monitoring vSphere installations. The vCenter Server Appliance responds to CLI commands, requests from the vSphere Web Client, and API requests from custom clients. API clients can be written in a choice of several software languages.
The vCenter Server Appliance is managed by CLI, Web interfaces, or API requests. These requests help you manage appliance configuration, monitor resource usage, or back up and restore the vCenter Server instance. You can also use API requests to check the health of the vCenter Server installed in the appliance. This programming guide explains how to use the APIs that are available to manage the appliance.
For more information about the capabilities of the vCenter Server Appliance, see vCenter Server Appliance Configuration.