Understanding Outbound Messaging
Outbound messaging uses the notifications() call to send SOAP messages over HTTP(S) to a designated endpoint when triggered by a workflow rule.

After you set up outbound messaging, when a triggering event occurs, a message is sent to the specified endpoint URL. The message contains the fields specified when you created the outbound message. After the endpoint URL receives the message, it can take the information from the message and process it. To do that, you must examine the outbound messaging WSDL.