though these terms are pretty simple most of the people have still confuse with these terms.
SharePoint pages are rendered using physical page template and content from content database.
These pages are not customized. thus SharePoint can offer these pages to every requesting location simply by merging physical file template and content from content DB. these kind of files are named as Ghosted files.
Users can customize content pages (using SharePoint Designer or Visual studio) that may also change the physical template file. thus SharePoint can no longer keep the standard file and customize file in the physical location. thus SharePoint keeps changed template file also in the content database. These files are called as Unghosted Pages.
Reghosting is a concept of revert back to definition.