A group of technologies aimed at supporting a web of data, the way current WWW supports a web of documents (the original Web3). Lots of components, lots of projects, lots of contributors. Sometimes unclear exactly what the relationships are between SemWeb-related things, or just how SemWeb-related the things actually are. Very Internet :)
Important topics: RDF, OWL, SPARQL, RIF
cf Apache Jena, a Java framework for SemWeb applications. Interact with RDF, store triples, work with OWL ontologies
arachne-framework/aristotle: Data-oriented Clojure bindings for Jena
The Symbol Management Problem (Dorian Taylor)