Define the navigation menus, and route page requests to code based on URLs.
The Drupal menu system drives both the navigation system from a user perspective and the callback system that Drupal uses to respond to URLs passed from the browser. For this reason, a good understanding of the menu system is fundamental to the creation of complex modules.
Drupal's menu system follows a simple hierarchy defined by paths. Implementations of hook_menu() define menu items and assign them to paths (which should be unique). The menu system aggregates these items and determines the menu hierarchy from the paths. For example, if the paths defined were a, a/b, e, a/b/c/d, f/g, and a/b/h, the menu system would form the structure:
Note that the number of elements in the path does not necessarily determine the depth of the menu item in the tree.
When responding to a page request, the menu system looks to see if the path requested by the browser is registered as a menu item with a callback. If not, the system searches up the menu tree for the most complete match with a callback it can find. If the path a/b/i is requested in the tree above, the callback for a/b would be used.
The found callback function is called with any arguments specified in the "page arguments" attribute of its menu item. The attribute must be an array. After these arguments, any remaining components of the path are appended as further arguments. In this way, the callback for a/b above could respond to a request for a/b/i differently than a request for a/b/j.
For an illustration of this process, see page_example.module.
Access to the callback functions is also protected by the menu system. The "access callback" with an optional "access arguments" of each menu item is called before the page callback proceeds. If this returns TRUE, then access is granted; if FALSE, then access is denied. Menu items may omit this attribute to use the value provided by an ancestor item.
In the default Drupal interface, you will notice many links rendered as tabs. These are known in the menu system as "local tasks", and they are rendered as tabs by default, though other presentations are possible. Local tasks function just as other menu items in most respects. It is convention that the names of these tasks should be short verbs if possible. In addition, a "default" local task should be provided for each set. When visiting a local task's parent menu item, the default local task will be rendered as if it is selected; this provides for a normal tab user experience. This default task is special in that it links not to its provided path, but to its parent item's path instead. The default task's path is only used to place it appropriately in the menu hierarchy.
Everything described so far is stored in the menu_router table. The menu_links table holds the visible menu links. By default these are derived from the same hook_menu definitions, however you are free to add more with menu_link_save().