Restricting content access by content type is a broad application of the Contact Access module. What this means is that a permitted user designates a particular content type to be editable and/or viewable by only a certain subset of people.
An example of when this might be used would be if a group wanted to create some type of internal training documentation for the members of their site. A custom content type such as "Internal Training" could be created, for example, and content access could be restricted to only those roles that it applies to. In the default OSU Drupal 6 installation, this would be authors, advanced authors, and administrators.
The roles that can configure this are limited to advanced authors and administrators. Once it is set, it is applied to all nodes of that particular type.
In the following example, we'll restrict the Book page content type so that only authors, advanced authors, and administrators can view and edit the content - please note, though, that this can be done on any content type of your choice:
From this point on, any visitor who comes across the url of a Book page content type and is not an author, advanced author, or administrator on this particular site, will see the following message: