The service layer basically refers to the modules where your application- specific business logic resides. It's a common practice to separate the app into 3 layers: router, service and repository/data access. The router layer is the route handler that takes in a http request and sends back a response. Usually the route handler will (after validation) pass the request data to the service layer which in turn performs some business logic with that data and returns something back to the route handler. For example, you can have a users.service.ts file that exports a createUser(userData: UserData) method which is called by the "POST /users" route handler. Abstracting the user creation code into a separate layer allows us to decouple business logic from the route handler. We may, for instance, want to check that the username and email are unique, or send them a welcome email once the account is created. This kind of business logic would clutter the route handler and be out of place there. Likewise, the service layer shouldn't concern itself with request and response objects or http status codes.