docs(guide/services): explain services in plain language

See the discussion at http://docs.angularjs.org/guide/dev_guide.services.understanding_services#comment-1002821035

Closes #4055

docs/content/guide/dev_guide.services.understanding_services.ngdoc

@@ -11,6 +11,14 @@ another service) that depends on the service.  Angular's dependency injection su
 of the rest. The Angular injector subsystem is in charge of service instantiation, resolution of
 dependencies, and provision of dependencies to factory functions as requested.
 
+The purpose of a service factory function is to generate a single object or function that 
+represents the service to the rest of the application. That object or function will then be
+passed as a parameter to any other factory function which specifies a dependency on this service.
+
+Angular factory functions are executed lazily. That is, they are only executed when needed 
+to satisfy a dependency, and are then executed exactly once for each service. Everything which is
+dependent on this service gets a reference to the single instance generated by the service factory.
+
 Angular injects dependencies using "constructor" injection (the service is passed in via a factory
 function). Because JavaScript is a dynamically typed language, Angular's dependency injection
 subsystem cannot use static types to identify service dependencies. For this reason a dependent
@@ -22,7 +30,6 @@ The Angular web framework provides a set of services for common operations. Like
 variables and identifiers, the built-in services always start with `$` (such as `$http` mentioned
 above). You can also create your own custom services.
 
-
 ## Related Topics
 
 * {@link di About Angular Dependency Injection}