Apex Web Services and Callouts

Apex Code supports the ability to expose Apex methods as a Web service. This article will refer to this as 'Apex Web Services.' 

Apex also supports the ability to invoke external web services and this article will refer to this concept as 'Callouts.' 

The former is involved in creating a web service that a client can invoke, while the latter is invoking an external web service.

For Apex Web Services, Apex supports the ability to write Apex logic and expose the logic as a web service. Therefore an external application 

can invoke this Apex web service to perform custom logic.

With 'Callouts', where Apex invokes an external web service, Apex provides integration with Web services that utilize SOAP and WSDL, 

or HTTP services (RESTful services). Apex supports the importing of WSDLs to auto-generate the corresponding Apex classes. 

Additionally, Apex supports HTTP services to use HTTP Request and Response objects to invoke the external web service.