What is the difference between wsdl mode and non-wsdl mode web service?

i think most of the soap servers offer web services in both wsdl and non-wsdl mode (like Amazon does) for a client to make request. we know in wsdl mode client has to download the wsdl file to determine how to make a request for the service. but in non-wsdl mode, how the client learns to make a request for the service?