A service is a general-purpose entry point for keeping an app running in the background for all kinds of reasons. It is a component that runs in the background to perform long-running operations or to perform work for remote processes. A service does not provide a user interface.
Started services tell the system to keep them running until their work is completed. This might be to sync some data in the background or play music even after the user leaves the app. Syncing data in the background or playing music represent different types of started services, which the system handles differently:
Bound services run because some other app (or the system) has said that it wants to make use of the service. A bound service provides an API to another process, and the system knows there is a dependency between these processes. So if process A is bound to a service in process B, the system knows that it needs to keep process B and its service running for A. Further, if process A is something the user cares about, then it knows to treat process B as something the user also cares about.
Note: If your app targets Android 5.0 (API level 21) or higher, use the JobScheduler class to schedule actions. JobScheduler has the advantage of conserving battery by optimally scheduling jobs to reduce power consumption and by working with the Doze API.