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The W3C Web service definition encompasses many different systems, but in common usage the term refers to clients and servers that communicate using XML messages that follow the SOAP standard. Common in both the field and the terminology is the assumption that there is also a machine-readable description of the operations supported by the server written in the Web Services Description Language (WSDL). The latter is not a requirement of a SOAP endpoint, but it is a prerequisite for automated client-side code generation in the mainstream Java and .NET SOAP frameworks. Some industry organizations, such as the WS-I, mandate both SOAP and WSDL in their definition of a Web service.

The W3C defines a Web service (many sources also capitalize the second word, as in Web Service) as "a software system designed to support interoperable Machine to Machine interaction over a network." Web services are frequently just Web APIs that can be accessed over a network, such as the Internet, and executed on a remote system hosting the requested services.

The W3C Web service definition encompasses many different systems, but in common usage the term refers to clients and servers that communicate using XML messages that follow the SOAP standard. Common in both the field and the terminology is the assumption that there is also a machine readable description of the operations supported by the server written in the Web Services Description Language (WSDL). The latter is not a requirement of a SOAP endpoint, but it is a prerequisite for automated client-side code generation in the mainstream Java and .NET SOAP frameworks. Some industry organizations, such as the WS-I, mandate both SOAP and WSDL in their definition of a Web service.

Core specifications
The specifications that define Web services are intentionally modular, and as a result there is no one document that contains them all. Additionally, there is neither a single, nor a stable set of specifications. There are a few "core" specifications that are supplemented by others as the circumstances and choice of technology dictate, including:

SOAP
An XML-based, extensible message envelope format with "bindings" to underlying protocols. The primary protocols are HTTP and HTTPS, although bindings for others, including SMTP and XMPP, have been written.
Web Services Description Language (WSDL)
An XML format that allows service interfaces to be described along with the details of their bindings to specific protocols. Typically used to generate server and client code, and for configuration.
Universal Description Discovery and Integration (UDDI)
A protocol for publishing and discovering metadata about Web services that enables applications to find them, either at design time or runtime.

Most of these core specifications have come from W3C, including XML, SOAP, and WSDL; UDDI comes from OASIS.

Profiles
To improve interoperability of Web Services, the WS-I publishes profiles. A profile is a set of core specifications (SOAP, WSDL) in a specific version (SOAP 1.1, UDDI 2) with some additional requirements to restrict the use of the core specifications. The WS-I also publishes use cases and test tools to help deploying profile compliant Web Service.

Additional specifications
Some specifications have been developed or are currently being developed to extend Web Services capabilities. These specifications are generally referred to as WS-*. Here is a non exhaustive list of these WS-* specifications.

WS-Security
Defines how to use XML Encryption and XML Signature in SOAP to secure message exchanges, as an alternative or extension to using HTTPS to secure the channel.

WS-Reliability
An OASIS standard protocol for reliable messaging between two Web services.

WS-Reliable Messaging
A protocol for reliable messaging between two Web services, issued by Microsoft, BEA and IBM it is currently being standardized by the OASIS organization

WS-Addressing
A way of describing the address of the recipient (and sender) of a message, inside the SOAP message itself.

WS-Transaction
A way of handling transactions.

Some of these additional specifications have come from the W3C. There is much discussion around the organization's participation, as the general Web and the Semantic Web story appear to be at odds with much of the Web Services vision. This has surfaced most recently in February 2007, at the Web of Services for the Enterprise workshop. Some of the participants advocated a withdrawal of the W3C from further WS-* related work, and a focus on the core Web.

In contrast, OASIS has standardized many Web service extensions, including Web Services Resource Framework and WSDM.

Web services are a set of tools that can be used in a number of ways. The three most common styles of use are RPC, SOA and REST.

Remote procedure calls
Architectural elements involved in the XML-RPC.
Architectural elements involved in the XML-RPC.

RPC Web services present a distributed function (or method) call interface that is familiar to many developers. Typically, the basic unit of RPC Web services is the WSDL operation.

The first Web services tools were focused on RPC, and as a result this style is widely deployed and supported. However, it is sometimes criticized for not being loosely coupled, because it was often implemented by mapping services directly to language-specific functions or method calls. Many vendors felt this approach to be a dead end, and pushed for RPC to be disallowed in the WS-I Basic Profile.

Service-oriented architecture

Web services can also be used to implement an architecture according to Service-oriented architecture (SOA) concepts, where the basic unit of communication is a message, rather than an operation. This is often referred to as "message-oriented" services.

SOA Web services are supported by most major software vendors and industry analysts. Unlike RPC Web services, loose coupling is more likely, because the focus is on the "contract" that WSDL provides, rather than the underlying implementation details

Representational state transfer

Finally, RESTful Web services attempt to emulate HTTP and similar protocols by constraining the interface to a set of well-known, standard operations (e.g., GET, PUT, DELETE). Here, the focus is on interacting with stateful resources, rather than messages or operations.

RESTful Web services can use WSDL to describe SOAP messaging over HTTP, which defines the operations, or can be implemented as an abstraction purely on top of SOAP (e.g., WS-Transfer).

WSDL version 2.0 offers support for binding to all the HTTP request methods (not only GET and POST as in version 1.1) so it enables a better implementation of RESTful Web services. However support for this specification is still poor in software development kits, which often offer tools only for WSDL 1.1.