Apex has powerful built-in functionality to allow for real-time integration between the Force.com platform and an external application or service. I recently published an introductory article recently on the topic of Apex Web Services and Callout. The article is intended for developers getting started with Apex and want to learn how to write their own custom Apex web services or make real-time callouts to external web services.

Here is a brief snippet from the article to give you an overview of the article's content:

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.
The next section of this article will go into the syntax and some
examples of how to enable an Apex Web Service.

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. These concepts are covered in the Apex Callouts section.

If you want to learn more about
how to write Apex web services or integrate to an external web service through Apex 'callouts', please continue with the article posted here.