At their most basic level, menu links are plugins, but this time discoverable via a YAML discovery (instead of annotations like we are used to). To this end, the regular menu links are defined inside a module_name.links.menu.yml file and can be altered by other modules by implementing hook_menu_links_discovered_alter(). When I say regular, I mean those links that take you to menus. We will see next that there are also a few other types.

One of the reasons for the YAML discovery is that links are more definition than functionality, so they don't necessarily require classes, but can be represented through some lines inside a YAML file.

There are some important classes you should check out in this architecture--MenuLinkManager (the plugin manager) and MenuLinkBase (menu link plugins base class and which implements the MenuLinkInterface).

Menu links are, however, also content entities. The links created via the UI are stored as entities because they are considered content. The way this works is that for each created MenuLinkContent entity, a plugin derivative is created. We are getting dangerously close to advanced topics, which are too early to cover. However, in a nutshell, via these derivatives, it's as if a new menu link plugin is created for each MenuLinkContent entity, making the latter behave as any other menu link plugin. This is a very powerful system specific to Drupal 8.

Menu links have a number of properties, among which is a path or route. When created via the UI, the path can be external or internal or can reference an existing resource. When created programatically, you'll typically use a route.