Invoking HTTP Callouts

Apex provides several built-in classes to work with HTTP services and create HTTP requests like GET, POST, PUT, and DELETE.

You can use these HTTP classes to integrate to REST-based services. They also allow you to integrate to SOAP-based web services as an alternate option to generating Apex code from a WSDL. By using the HTTP classes, instead of starting with a WSDL, you take on more responsibility for handling the construction of the SOAP message for the request and response.

The Google Data API Toolkit makes extensive use of HTTP callouts.