The top-down and bottom-up web services differ in the way they are created.
When a web service is created using top-down approach, the WSDL file is created first. The Java skeleton classes and interfaces are generated from the WSDL file using tools. Then the web service is implemented in Java.

When a web service is created using bottom-up approach, the web service is defined in the Java class. The WSDL file is generated from the Java class.
The top-down approach is recommended way of creating web service, but the bottom-up approach is faster and easier.