RDF or Resource Description Framework is a generic model for describing objects and data.
RDF/XML is a common representation, and unfortunately is intimidating, miserable to work with by hand, and just barely gets by as human unreadable (it's designed for machine consumption).
RDF/N-triples (aka RDF/N3) is a quick-and-easy representation by Tim-Berners Lee. He has a beginner's N3 primer.
Turtle is another natural language-looking, N3-like syntax by Dave Beckett.
RDF/JSON is a description for RDF in JSON, perfect for use in browser-based web applications. Freebase also uses some kind of RDF-in-JSON representation, though different.
Dave Beckett has a summary on various RDF representations in 2010.