A web service (WS) is either: a service offered by an electronic device to another electronic device, communicating with each other via the Internet, or a server running on a computer device, listening for requests at a particular port over a network, serving web documents (HTML, JSON, XML, images).
In a web service, a web technology such as HTTP is used for transferring machine-readable file formats such as XML and JSON.
In practice, a web service commonly provides an object-oriented web-based interface to a database server, utilized for example by another web server, or by a mobile app, that provides a user interface to the end-user. Many organizations that provide data in formatted HTML pages will also provide that data on their server as XML or JSON, often through a Web service to allow syndication. Another application offered to the end-user may be a mashup, where a Web server consumes several Web services at different machines and compiles the content into one user interface.
Web services (generic)
Asynchronous JavaScript And XML
Asynchronous JavaScript And XML (AJAX) is a dominant technology for Web services. Developing from the combination of HTTP servers, JavaScript clients and Plain Old XML (as distinct from SOAP and W3C Web Services), now it is frequently used with JSON as well as, or instead of, XML.
REST
Representational State Transfer (REST) is an architecture for well-behaved Web services that can function at Internet scale.
In a 2004 document, the W3C sets following REST as a key distinguishing feature of Web services:
Web services that use markup languages
There are a number of Web services that use markup languages: JSON-RPC. JSON-WSP Representational state transfer (REST) versus remote procedure call (RPC) Web Services Conversation Language (WSCL) Web Services Description Language (WSDL), developed by the W3C Web Services Flow Language (WSFL), superseded by BPEL Web template WS-MetadataExchange XML Interface for Network Services (XINS), provides a POX-style web service specification format
Web API
A Web API is a development in Web services where emphasis has been moving to simpler representational state transfer (REST) based communications. Restful APIs do not require XML-based Web service protocols (SOAP and WSDL) to support their interfaces.
W3C Web services
In relation to W3C Web services, the W3C defined a Web service as:
W3C Web Services may use SOAP over HTTP protocol, allowing less costly (more efficient) interactions over the Internet than via proprietary solutions like EDI/B2B. Besides SOAP over HTTP, Web services can also be implemented on other reliable transport mechanisms like FTP. In a 2002 document, the Web Services Architecture Working Group defined a Web services architecture, requiring a standardized implementation of a "Web service."
Explanation
The term "Web service" describes a standardized way of integrating Web-based applications using the XML, SOAP, WSDL and UDDI open standards over an Internet Protocol backbone. XML is the data format used to contain the data and provide metadata around it, SOAP is used to transfer the data, WSDL is used for describing the services available and UDDI lists what services are available.
A Web service is a method of communication between two electronic devices over a network. It is a software function provided at a network address over the Web with the service always-on as in the concept of utility computing.
Many organizations use multiple software systems for management. Different software systems often need to exchange data with each other, and a Web service is a method of communication that allows two software systems to exchange this data over the Internet. The software system that requests data is called a service requester, whereas the software system that would process the request and provide the data is called a service provider.
Different software may use different programming languages, and hence there is a need for a method of data exchange that doesn't depend upon a particular programming language. Most types of software can, however, interpret XML tags. Thus, Web services can use XML files for data exchange.
Rules for communication with different systems need to be defined, such as: How one system can request data from another system. Which specific parameters are needed in the data request. What would be the structure of the data produced. (Normally, data is exchanged in XML files, and the structure of the XML file is validated against a .xsd file.) What error messages to display when a certain rule for communication is not observed, to make troubleshooting easier.