Web services is all about give and take data over HTTP. When you give data you are a 'provider' of a web service and when you take data you are a 'consumer' of a web service.
Oracle 11g is doing both.
You can turn your Oracle database into web service provider, a source of data for the internet where people can come to a certain URL (WSDL) and be able to query data that you keep in your database, just using HTTP. The visitors to this URL will run your database PL/SQL procedures, functions and packages using this URL and then they will be able to read the results. This setup is called being a provider of a web service.
On the other hand, being a consumer of a web service in Oracle, is being able to write SQL or PL
/SQL which makes external calls from your Oracle database to other web service providers' services on the internet. I have used this type of webservices in my blog. An example of such a web service is when you make an HTTP_REQUEST to the Google Maps API or other sources such as Yahoo and PayPal which themselves are providers of web services. See this demo app
When your database makes a call to an external web services then you become the consumer of a web service.
A platform to try both, with no hassle, of course is Oracle Apex. The installation of Apex in Oracle 11g will enable you to use web services in both ways described above.
The best place to start is Chapter 33 of Oracle XML Developers Guide. Here you will find how to setup and get Oracle 11g to use Native Oracle XML DB web services. Once you set up, all you have to do really is create a PL/SQL procedure or function and the WSDL is automatically generated and saved in Oracle for you. All you have to do then is just run the procedure from the URL via a little program which will call this web service, yes you can use 'wget' or any other command line script which will make a call to web service.
The next link is a good blog entry which describes step by step how to setup Native Oracle XML DB Web Services and how to Access PL/SQL stored Procedures Using a Web Service in Oracle. The post is called the First Tests of 11g native web services by Paul Gallagher I really liked Paul's post, is concise and straight to the point. It even has a perl script which tests to see if your web service provider database setup works.
Other related posts on the topic of Oracle web services are:
Create a Native Database Web Service by Marco Gralike