Service Applications are actually nothing more than a logical concept. There is actually no such thing as a “Service Application”. Reference and for more information, click here.
The Service Application makes Service Application Services available like Excel Services, Managed Metadata Service or Search Service to web application that connects to it by Service Application Connection (Proxy). Actually they are just covers for Service Application Services so that we can use these services. A web application can consume service(s) from one or many Service Application(s). A Service Application can be provide service to one or many Web Application(s).
Here you can watch the presentation about Service Applications.
Some Service Applications Services need database to store information. For example, Search service application requires a database to work (for indexing purposes) but Excel Services don`t need a database. On the other hand Excel Services may need more CPU power than the Search Service or Managed Metadata Service.
Here is a Technet Virtual Lab about this topic.
- Creating a service application creates the service application proxy that facilitates web service calls between the Web Front End (WFE) and the application server using the Windows Communication Framework.
- A unique name and an application pool are required for configuring a new service application. The name is the display name of the service, and differs from the service application`s identity, whicch is a GUID that is automatically assigned when the application is created.
- As a part of creating a new service application you have the option to specify whether the service application instance should be available by default for use by web applications in the farm.
- A default application proxy is used by all service applications in a given web application to communicate with that service application`s web service. For a proxy to be the default of the default group, it must be designated as such on the Service Application Associations page.
- Only one procy group can be associated with a web application. Within that proxy group, you can enable as many proxies as necessary.