What modules do I need to implement web services? We have planned to create a bank loan application in Drupal. The Application does not have any own users records, but it completely depends on external service provider. 

For example, when we log in to the application, the username and password should hit the service provider, and fetch the result from the provider; the login information should be validated into the application.

How do I implement the web service in Drupal? What type of modules do I need to use? 

I need to be able to do CRUD operations using web services.
I've used the soapclient module in D6 for my cause with a SOAP server. It works by making SOAP calls and keeps things really simple. One method to instantiate the SOAP object and another to make the call. It has a good test page where you could enter the webservice end point, the mode like XML RPC etc. and other options such as the service method and service parameters and test the response which is quite good as you can test the service without any actual code. If you plan on implementing webservice with SOAP it is a nice module.