What is the purpose of sites.php and how does it relate to settings.php? What is the purpose of sites.php and how does it relate to settings.php?

I have done a lot of web searches and looked at the comments in these files, but cannot find a simple explanation.
Looking at conf_path() should help clarify the relationship here. conf_path() builds the default subdirectory (inside the sites directory) Drupal will goto for settings.php (and other site-specific configurations). It's default search behavior can be overridden with sites.php.