Contextual information can be drawn from many sources when content is displayed by Drupal including the content node, fields, taxonomies, and user viewing the site. Modules such as Page Manager, Panels, and Views use "Contextual Filters" to obtain this information from the site database and use it to refine the contents and display according to settings provided by the site developer. Other module may also use them by expanding upon the functionality provided by these base modules. Many web tutorials and books are available regarding Page Panels and Views to detail how they work and can be used.
- Taming the Beast from nodeone.se is an excellent video series that devotes six episodes to working with contextual filters in the Views Module. The first episode links to the rest of the series.