The concept of semantic services is about 10 years old.   The B2B movement was a major driver in this aspect of overall service-oriented architecture ( SOA ).  Much of the foundations of the Semantic Web vision is the result of the fruition of the extensive work done by the W3 Consortium to create standards for interoperability between different web services.

The term 'Semantic Web' has almost become synonymous with 'Semantic Services', but that is not really correct.  Semantic services may or may not be used to support a semantic web, which contains more a sense of knowledge sharing and associated knowledge communities than it does the technical infrastructure supporting those communities.  As an analogy, there is a distinct difference between the knowledge compiled in a Wiki and the supporting 'Wiki' technology which facilitates it.  Wiki software can be used for a variety of purposes, for example to create a quick and dirty web site.  

Semantic services are the foundation or technical infrastructure of the Semantic Web.   The historical sequence of standards and facilities is an almost exact bottom up in the architecture of semantic services.  

Ten years ago, the term Semantic Services referred to a fairly low level of functionality compared to a knowledge-intensive approach of the Semantic Web.  In addition to meta-data and database schema, semantic services included now-familiar base technologies such as XML and XMI to allow for the free interchange and conversion of data or modeling information between different formats. 

The idea of semantic services had a more document-oriented outlook than the Semantic Web, primarily intended to create, parse, translate and manipulate documents in various ways and provide document composition services and languages in general.  The term could also refer to the distribution and integration infrastructure, such as as object life-cycle management facilities and even the CORBA technology required for controlling objects in a distributed operating environment.

Another important component of semantic services was to provide a mechanism for the discovery of other services.  This of course will be critical for the Semantic Web.

Recently developed features of Semantic Services are more tuned to the Semantic Web and demonstrate an emerging sense of semantics services as more than documents, as something to capture and disseminate knowledge.  It's distinctly more "rule-based" than in the past. 

A major standard is the Resources Description Framework ( RDF ), representing facts and assertions about resources on the web, possibly resources as mundane as URL links to shopping sites or MP3 downloads.  It could also be, for example, a set of convoluted qualification rules for the latest changes Medicare benefits, with a working spreadsheet so an individual could see how the rules would effect their medical costs.  In other words, it could be a knowledge resource.

There are several competing standards and areas of overlap, but they all share more similarities than differences.              

A shared ontology must have a shared language ( Latin in the case of animals ).  On the web, it is called the Web Ontology Language or OWL ( speaking of animals ).  The Wikipedia defines OWL as;

... a markup language for publishing and sharing data using ontologies on the Internet. OWL is designed for use by applications that need to process the content of information instead of just presenting information to humans. OWL facilitates greater machine interpretability of Web content ... by providing additional vocabulary along with a formal semantics.