Actually every item is well explained inline within the code.
Both RequestBase and ResponseBase are only base classes that real messages derive from. A message is an item that is sent between the server and the client (the client requests and the server responds, e.g.) by using a webservice (in this case). To standardize the format in which the messages are sent back and forth, every message must derive from respective base class.
The concrete implementation of a specific message then contains the properties that reflect its own actual meaning.
Hope that helps.
Christian Jacob, Jun 21, 2010