Message and Transport Security for Web Services 
You can use transport security for consuming and providing Web services either at HTTP transport level or at SOAP message level. To do this, you can use the standard HTTP transport security mechanisms such as HTTPS and the WS standard mechanisms to set the transport security at the higher SOAP message level.
Transport security at SOAP message level is suited to the specific security requirements for WS access, and also allows you to use the strong SOAP message transport mechanisms such as XML signatures/encryption and SecureConversation for inbound and outbound WS communication.
More information: