A Web service is a set of functionalities that are packaged as a single entity and published to the Web for use by other applications. Web services are building blocks for creating open distributed systems, and allow companies and individuals to quickly and cheaply make their digital assets available worldwide. One early example is Microsoft Passport, a convenient authentication service hosted by Microsoft. Here are some other examples:

• a credit checking service that returns credit information when given a person's social security number.
• a stock quote service that returns the stock price associated with a specified ticker symbol.
• a purchasing service that allows computer systems to buy office supplies when given an item code and a quantity.

A Web service can aggregate other Web services to provide a higher-level set of features. For example, a Web service could provide a set of high-level travel features by orchestrating lower-level Web services for car rental, air travel, and hotels. Applications of the future will be built from Web services that are dynamically selected at runtime based on their cost, quality, and availability.
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