For questions regarding the use and/or implementation of web services. For general questions about web services, consider asking at Web Applications Stack Exchange.

Web services are a standardized way of integrating Web-based applications, using various open standards over an Internet protocol backbone, such as Web Services Description Language (WSDL) and Universal Description, Discovery, and Integration (UDDI).

The W3C definiton:

A Web service is a software system designed to support interoperable machine-to-machine interaction over a network.