Semantic Web Services, like conventional web services, are the server end of a client–server system for machine-to-machine interaction via the World Wide Web. Semantic services are a component of the semantic web because they use markup which makes data machine-readable in a detailed and sophisticated way (as compared with human-readable HTML which is usually not easily "understood" by computer programs).
The mainstream XML standards for interoperation of web services specify only syntactic interoperability, not the semantic meaning of messages. For example, Web Services Description Language (WSDL) can specify the operations available through a web service and the structure of data sent and received but cannot specify semantic meaning of the data or semantic constraints on the data. This requires programmers to reach specific agreements on the interaction of web services and makes automatic web service composition difficult.
Semantic web services are built around universal standards for the interchange of semantic data, which makes it easy for programmers to combine data from different sources and services without losing meaning. Web services can be activated "behind the scenes" when a web browser makes a request to a web server, which then uses various web services to construct a more sophisticated reply than it would have been able to do on its own. Semantic web services can also be used by automatic programs that run without any connection to a web browser.
A directory of Semantic Web Services providing semantic web service investigators with an index to projects, standards and bibliographical references of semantic web service proposals was created and is maintained by Dr. Khalid Belhajjame.
A Semantic Web Services platform that uses OWL (Web Ontology Language) to allow data and service providers to semantically describe their resources using third-party ontologies is SSWAP: Simple Semantic Web Architecture and Protocol. SSWAP establishes a lightweight protocol (few OWL classes and predicates; see the SSWAP Protocol) and the concept of a "canonical graph" to enable providers to logically describe a service. A service is essentially a transformation of some, possibly null, input (or subject) to some, possibly null, output (or object). Services are semantically discoverable based on their subsumption hierarchies as well as their input and output data types.
Choreography vs. orchestration
Choreography is concerned with describing the external visible behavior of services, as a set of message exchanges optionally following a Message Exchange Pattern (MEP), from the functionality consumer point of view.
Orchestration deals with describing how a number of services, two or more, cooperate and communicate with the aim of achieving a common goal.