When using web services, you cannot pass objects from the service to clients or in the other direction.
Think of it as passing a representation of some data from client or server to the other party. Data to be sent is "translated" (marshalled) to XML (or some other representation format, like JSON). Marshalling involves querying one or more objects of their state and generating a representation.
When arriving at the recipient, the representation is "translated" (unmarshalled) back to data. Unmarshalling involves creation of new objects and setting their state according to the representation.