For a decent explanation of what WSI is and how it would be used in the real world, have a look at this forum thread.
If you want an example of a simple desktop application that can pass and consume xml to and from the web-service, you can download the example application here or from the Sample Test Program page in the AspDotNetStorefront WSI manual. You can open the files included in the test program to see how the connection is made and how xml is consumed by the web service and then rendered and displayed in the tool.
For examples of how to build xml in code, you can have a look at our ERPS specification pdf that we link to from the AspDotNetStorefront manual.
I'm happy to provide a specific example as well, but I think it's prudent that you review all of the available resources and documentation available for WSI to help you understand exactly what WSI is and what it's capable of first
<a href="http://www.aspdotnetstorefront.com">Shopping Cart Software</a>