1. An injector is a service locator. 2. It is used to retrieve object instances as defined by provider, instantiate types, invoke methods and load modules. 3. There is a single injector per Angular application, it helps to look up an object instance by its name.
In angular.js services are the singleton objects or functions that are used for carrying out specific tasks. It holds some business logic and these function can be called as controllers, directive, filters and so on.