How will it be different to the web we know now?
Right now the HTML+CSS is centered more on structure and presentation. Semantics is about the meaning of the information. In semantic web you use shared ontologies to establish meaning (semantic) of the object and meaning of relations between the objects.
Best known ontologies are: FOAF and Dublin Core.
Typically semantics would be expressed in specialized language, such as RDF or OWL. RDF can be embedded within XHTML using eRDF or W3C's RDFa.
Less structured alternative to eRDF/RDFa are microformats.
Read more at:
http://en.wikipedia.org/wiki/Semantic_web
