RDF resources have properties, and properties have values. RDF data takes the form of a collection of statements of the form "The value of property P on resource S is O". (S is called the subject of the statement and the value O the object of the statement). Values of properties may be literals (such as numbers or strings), or other resources. For example, a town such as Astoria (in Oregon,USA) can be represented by an RDF resource, and the property "latitude of" by an RDF property. The value of this property on this resource is the literal value 46.183. A particular property on a resource may have more than one value.
In mathematical terminology, the RDF data model is a graph, whose nodes are resources and literals, and whose edges are properties. Properties are also a kind of resource, so that properties can themselves have properties.
Resources often have names, though they can be anonymous as well. The name of an RDF resource (if it has one) is a URI (uniform resource identifier) which identifies that object uniquely in the world, just as the URL of a web page is a unique name for that page. A URI is a generalization of the notion of a URL (uniform resource locator); the difference is that a URL locates an object on the web so that software can access it using web protocols, whereas a URI is only required to name the object unambiguously. URLs only apply to things which reside on the web (files, servers, etc), whereas URIs can be used to name anything at all. The syntactic rules for URIs are the same as those for URLs.
If the subject S and predicate P of an RDF statement are named by URIs s and p, and its object O is either a literal L, or named by URI o, then the statement can be represented by the triple:
<s> <p> L .
or by
<s> <p> <o> .
respectively. This is the simplest syntax for RDF, called N-Triples. A statement is a mathematical object, and a triple is one way to write it down. See the RDF Semantics page for a detailed account of this distinction.
Here is an example:
http://example.org/places/us/oregon/astoria
is a URI which names the town of Astoria in the state of Oregon, within the US. Note carefully that http://example.org/places/us/oregon/astoria does not name a web page about Astoria, it names the geopolitical entity Astoria itself. Now,
http://www.w3.org/2003/01/geo/wgs84_pos#lat
names a property meaning "is the latitude of" in the RDFIG Geo vocabulary. Then, the triple:
<http://example.org/places/us/oregon/astoria> <http://www.w3.org/2003/01/geo/wgs84_pos#lat> 46.183 .
represents a correct statement about the town of Astoria.
In this manual, the notation <s,p,o> is sometimes employed to abbreviate:
<s> <p> <o> .