Wikipedia: http://en.wikipedia.org/wiki/Semantic_technology 

With traditional information technology, on the other hand, meanings and relationships must be predefined and “hard wired” into data formats and the application program code at design time. This means that when something changes, previously unexchanged information needs to be exchanged, or two programs need to interoperate in a new way, the humans must get involved.

Off-line, the parties must define and communicate between them the knowledge needed to make the change, and then recode the data structures and program logic to accommodate it, and then apply these changes to the database and the application. Then, and only then, can they implement the changes.

Semantic technologies are “meaning-centered.” They include tools for:

• autorecognition of topics and concepts,
• information and meaning extraction, and
• categorization.

Given a question, semantic technologies can directly search topics, concepts, associations that span a vast number of sources.

Semantic technologies provide an abstraction layer above existing IT technologies that enables bridging and interconnection of data, content, and processes. Second, from the portal perspective, semantic technologies can be thought of as a new level of depth that provides far more intelligent, capable, relevant, and responsive interaction than with information technologies alone.