At their most basic level, menu links are YAML-based plugins (like the Layout plugins we saw in the previous chapter). To this end, 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 go into menus. We will see shortly that there are also a few other types.
There are a number of important classes you should check out in this architecture though: MenuLinkManager (the plugin manager) and MenuLinkBase (menu link plugins base class and which implements MenuLinkInterface).
Menu links can, however, also be 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 that are too early to cover. But 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 a path or route. When created via the UI, the path can be external or internal or can reference an existing resource. When created programmatically, you'll typically use a route.