What are the differences between Metro WSIT's Transport Security and Message Authentication over SSL?
The Transport Security mechanism protects your application during transport using SSL for authentication and confidentiality.
Message Authentication over SSL:
The Message Authentication over SSL mechanism attaches a cryptographically secured identity or authentication token with the message and use SSL for confidentiality protection.
Authentication refers to the security layer designed to confirm a person's identity (via a username and password, for example).
Transport security refers to the security layer that protects the transmission of information, including authentication credentials (username & password), and any data transmitted during use of the application.
To say it another way - authentication is the "ID check" to make sure you are who you say you are, and transport security is what keeps your transactions a secret that is shared only with you, and the server and/or service you are interacting with.