DBpedia is a community effort to extract structured information from Wikipedia and to make this information available on the Web. DBpedia allows you to ask sophisticated queries against Wikipedia and to interlink other datasets on the Web with DBpedia data.
Information about the DBpedia project and dataset can be found here:
DBpedia on MetaWiki 
Several pages were created on MetaWiki to aid in extracting information from Wikipedia. Semi-structured information can be found on Wikipedia in articles with infoboxes. The main problem is that infoboxes are generally designed to appeal to human users, when rendered. For machines it is much harder to access the information in infoboxes and extract meaningful and useful data. To ease this process two different pages were created in the User:DBpedia-Bot namespace:
DBpedia Ontology 
The DBpedia Ontology aims to be a common scheme for articles (and especially infoboxes of articles) in Wikipedia. It is a repository of unique names and identifiers (vocabulary) for the strings used in infoboxes and aims to merge together what belongs together and separate strings with different meaning.
A simple example: On the English Wikipedia's infoboxes there are 27 different variants of birthPlace such as birthplace, placeOfBirth, born, which all have the same meaning, i.e. telling the place where a person was born. On the other hand, in Wikipedia:Bjork for example born is used for birthDate and birthPlace at the same time.
The DBpedia Ontology provides a single name for each meaning and thus makes it easier for machines to extract information correctly.
Besides names for properties it also contains classes, which provide a strict categorisation system for articles.
DBpedia Mapping 
Each infobox template (such as Wikipedia:Template:Infobox_Musical_artist) can be assigned an OWL class from the ontology. Furthermore, each template parameter (such as Background, Born, Died, Origin) can be mapped to an ontology property.
More information about the can soon be found on the mapping page.