What is a Sharepoint site definition? What is ghosted (uncustomized) and unghosted (customized)?

Sharepoint site definitions are the core set of functionality from which Sharepoint site are built from, building from the SiteTemplates directory in the Sharepoint 12 hive. Site definitions allow several sites to inherit from a core set of files on the file system, although appear to have unique pages, thereby increasing performance and allowing changes that happen to a site propagate to all sites that inherit from a site definition. Ghosted means that when Sharepoint creates a new site it will reference the files in the related site definition upon site provisioning. Unghosted means that the site has been edited with an external editor, and therefore the customizations are instead stored in the database, breaking the inheritance of those files from the file system.