Service Application API
A service that provides the core business logic of the service. It usually has no UI. You can have multiple instances of your service running across multiple servers but only a single instance of your service on any one server. The SPService object has an Instances property that holds all the instances of the service that are running on various servers in the farm. Each instance is represented by an object of a class derived from SPServiceInstance.
SPServiceInstance represents an instance of a service on a server in the farm. Just like Window Services, your service instances may be in different states such as started or stopped. An instance is the host process for the service application.
A service application is usually created by an administrator by configuring and running your service. That service is then hosted by a service instance.
An SPServiceProxy is the parent of the SPServiceApplicationProxy.
Service application proxies are the public interface for your service application. Through the proxy, other services or programs can interface with your service application.