A web service has the following characteristics:

1. It communicates using open protocols like HTTP
2. Processes XML messages framed using SOAP
3. May be described using WSDL
4. Can be discovered using UDDI

Any application which is supposed to reach a wide customer base should be written as a web service as opposed to customized applications for specific customers.

For example services which help in stock trading by providing analysis of market trends could best be implemented as a web service. The reasons are

1. Such a web service would be easily discovered by potential customers when they need it.
2. It will require minimal setup on the client machines
3. The updates to the system will be automatically available to all its consumers without any need to deploy the updates on their machines.