Stable Class
What it does
Adds router directives and providers.
How to use
RouterModule can be imported multiple times: once per lazily-loaded bundle. Since the router deals with a global shared resource--location, we cannot have more than one router service active. That is why there are two ways to create the module: RouterModule.forRoot and RouterModule.forChild.
forRoot creates a module that contains all the directives, the given routes, and the router service itself.
forChild creates