A message in WSDL is data structure that is used to pass information in and out of SOAP operations.

The message can be simple or complex. Complex messages consists of several parts, where each part refers to a type. This allows a designer to identify message components (base data structures) and define them as types. Then, the components can be combined to form message structures.

If you had a common type (like phone number), you could define it once and then reuse it in multiple messages (customer contact, employee contact, etc.) In this manner, messages become the ultimate entity that you want to pass around and types become the building blocks for the message.

Messages are referenced by a Port Types operation (input and output message).