ontology representation

Can I represent onto开发者_运维技巧logy by using XML? Why it is considered better to represent ontology by RDF?

If you use RDF, then all of the reasoning engines (Jena is my favourite) and triple stores out there can help you. If you want to choose your own format, you won't be able to use any other software that implements the standard.

Yes, an ontology can be described using XML, see http://en.wikipedia.org/wiki/RDF/XML
RDF is pretty old and has mature tools, see also Web Ontology Language

In many ways, the answer to your question is yes. There are a number of ontology languages which can be serialised in XML. On the web, Web Ontology language is the tool of choice as has been pointed out (which can be serialised in XML). Whether RDF (which can also be serialised in XML) can be considered to be an ontology language is contentious: RDF, unlike OWL, does not give you access to the whole apparatus of first order description logic, but allows you to construct things more akin to controlled vocabularies, taxonomies etc...rather than formal ontologies. Therefore the question of what it is that you mean by "ontology" is actually quite justified: people often use the term ontology when they actually mean "dictionary" or "controlled vocabulary" and others for formal axiomatic systems. Depending on what you want to do, you may even find that first order logic is not enough for you and you may have to go to other (higher order) formalisms.

If by "ontology" you mean OWL then check out the W3C Recommendation OWL 2 Web Ontology Language XML Serialization. This XML-based (and not RDF-based serialization) is in many cases more suitable for representing OWL ontologies than any RDF-based syntax, e.g. it is easier to write by hand, more straight-forward to manipulate automatically, etc.