OWL

From GetSemantic

OWL (Web Ontology Language) is a Semantic Web schema vocabulary which allows you to describe specific restrictions on how RDF classes and property relationships are used, specifically:

class disjointness - mutual incompatibility - for instance, if something is a 'Dog', it can not be a 'Cat'
cardinality - the number of times that a property may appear - for instance, one might specify that the class 'Car' can only have one 'Wheels' property - :car :wheels "4"; :car :wheels "3". thus being invalid.
enumeration - listing the possible options that may be allowed for a particular property, much as enumeration works in XML schema languages like XSD and RELAX NG
symmetry - "if John is married to Jane, Jane must be married to John"
transitivity - "if I know how to contact John, an John knows how to contact Jane, then I know how to contact Jane"
inverse relations - "if John has a wife Jane, then Jane has a husband John"

OWL also introduces a concept that was not available in RDFS - the distinction between object properties and datatype properties. The former are properties that must link to another object. So, for instance, if one has a class called 'Family', and it has a property called 'hasMember', then 'hasMember' must hold an object, whereas a datatype property could hold a string literal instead.

1 Should I use OWL?
2 Tools
3 See Also
4 Links

[edit] Should I use OWL?

In most cases, the complexity of OWL may not be necessary for many instances. Simple schemas to be used on the web often do not require the use of either an RDFS schema or an OWL ontology. But if one is making the choice to build a machine-readable specification, OWL seems to be the most sensible route.