From the business perspective: A Web service is a software asset that automates a task and can be shared, combined, used, and reused by different people or systems within or among organizations.

From the information systems perspective: A Web service is a software system designed to support interoperable machine-to-machine interaction over a network. It has an interface described in a machine-processable format (specifically, WSDL). Other systems interact with the web service in a manner prescribed by its description using SOAP or REST messages, typically conveyed using HTTP or HTTPS with corresponding JSON-formatted or XML-serialized text files.

What is a Decision Service?

A Decision Service automates a discrete decision-making task. It is implemented as a set of business rules and exposed as a web service. By definition, the rules within a Decision Service are complete and unambiguous; for a given set of inputs, decision integrity is ensured because the Decision Service addresses every logical possibility uniquely.

What is a web services caller?

A web services caller is a software application that makes a request to, and receives a response from, a web service. Most modern application development environments provide native capabilities to call web services.