​A Service is a singleton object with a lifetime scope of the application.  It is instantiated only once during the lifetime of an application.  When combined with Angular’s Dependency Injection, a service can be injected into any component in an application via Constructor Injection. This makes a Service perfect for sharing reusable functionality throughout an application.

A common example of when you’d use a Service is when you want to retrieve data from a WebApi using the HttpClient. There may be several places in your application where you need to retrieve the same list of data. By placing this functionality in a Service it gets rid of the duplicated code in the components that make the WebApi call.

Figure: Bad Example - Code that is reusable should be placed in a Service

Figure: Good Example - ​ Reusable code is placed in a Service and the component calls the Service