In Business Suite Application Framework, Controllers determines how HTTP requests should be handled.

A Controller is simply a php file inside system/controllers/ directory that is named in a way that can be associated with a URI.

Controllers are stored in the system/controllers/ directory.

Consider this url-
Or [ If url_rewrite is enabled ]

In the above example, Framework would attempt to find a controller named invoices.php in system/controllers/ and load it.

It can be visually presented like this-