Difference between RPC-literal and document- literal.

When we expose a webservice, we have an option to select RPC-literal or document- literal.
I came eto know the difference form the post
http://www.wmusers.com/forum/showthread.php?8684-Best-practices-for-creating-web-services-using-webMethods-IS

But i am not able to undertand the point interoperability with the messages is good in document literal. as when we expose our webservices to the external system they will pass the input fields required for us. we will use the value of the fields for our business logic and then we update them in return with a response.

This can be done in both RPC-literal and document- literal.I am not able to make difference

Please guys help me in sorting it out.

Thanks
Bharath