Web services are a new breed of Web application. They are self-contained, self-describing, modular applications thay can be published, located, and invoked across the web. Web services perform functions, which can be anything from simple requests to complicated business processes … Once a web service is deployed, other applications (and other web services) can discover and invoke the deployed service
IBM Web Service Tutorial
A distributed system is a collection of independent computers that appear to the user as a single coherent system, this is in contrast to a networked system. In a networked system computers exchange information, however in a distributed system an application has many parts running on different computiers, and information is shared to accomplish a specific purpose.
Distributed Systems allow resources to be more easily shared, to be location independant, to distribute human resources, to increase performance, to increase modularity and scalability and to increase availability and reliability.