As I am researching in the field of the semantic web and ontologies, I come across a lot of sites covering these topics. This post is intended to collect useful links and organize them.
The following sites are ideal as starting point. Or for reference.
Introduction to Ontologies and Semantic Web
This site gives a good introduction to both concepts. It explains what ontologies are, what they can be used for and how they can be represented. The semantic web is used as a working example for ontologies and their benefits.