A web service runs on a Microsoft Web Server and, if you reference it from a
VB.NET application (windows, web, whatever) you can use it like a class and create an instance of it and use its methods etc.....
A good example is Amazon have a web service that people can use to query their database of books, so somebody with a website about say gardening could use Amazons web service to provide information about gardening books and they dont have to have their own database of books.
Behind the scenes, the data is transported in XML format and requests are made using SOAP, but if you probably dont need to know that for creating simple Web Services and consuming them from other applications.
Do a search on
www.google.com for "XML Web Services
VB example" and you should get plenty to get you started.
HTH