some basic ontology problems, need for more explanation

dear all
actually i am quite confused about ontology and some basic concepts related to it.
i appreciate if someone could give me a more clear explanation of it.
i did search on internet about the concept, the best one i can is
http://www-ksl.stanford.edu/kst/what-is-an-ontology.html
the stanford fella defined that "An ontology is a specification of a conceptualization"
Thru all what i read, what i can understand is  that ontology seems to be some definition or
description of some concept, or the relation between/among concepts. (very abstract.)
so in that case, 
1) how can it related to knowledge sharing? and 
2)how can Ontology related to Semantic Web(did i ask this question before?)
Semantic Web is actually fully based on RDF, and RDF is mainly XML codes.
and 
3)how can these XML codes related to Ontology? 
if you think i am too naive then i am very sorry to ask all these questions, just hope
you could give me a clear guide. 
 
thanks zillions and kind regards
urs.
Sandy
 



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