MENU_CALLBACK -- A hidden, internal callback, typically used for API calls.
MENU_NORMAL_ITEM -- A "normal" menu item that's shown in menu and breadcrumbs.
MENU_DEFAULT_LOCAL_TASK -- The "default" local task, which is initially active.
MENU_LOCAL_TASK -- A task specific to the parent item, usually rendered as a tab.
MENU_LOCAL_ACTION -- An action specific to the parent, usually rendered as a link.
MENU_SUGGESTED_ITEM -- A normal menu item, hidden until enabled by an administrator.