Services are the components that make an application run in the background for various kinds of operations. These services don’t require user interaction with the application to keep them running. They just use callback methods to implement two different states of an application such as Started and Bound. 
To create an Android service, you need to create a Java class that extends the Service base class. There are several base class callback methods in Java that include the following:

• onBind(): This method is called when a component requires to bind with the service. It is implemented by calling the bindService() method. When you implement this method, you need to provide an interface for client interaction with the help of the IBinder object. 
• onUnbind(): This method is called when the user disconnects from the application or a specific interface of the app. 
• onStartCommand(): It is called by the system when a service is requested by a component. The service is called using the startService() method. 
• onCreate(): This is called when a service is created by onStartCommand() or onBind() method. This method is required to be called to perform a one-time set-up. 
• onDestroy(): The system calls this method when there is no longer needed or destroyed by the component.