Android service is a component that is used to perform operations on the background such as playing music, handle network transactions, interacting content providers etc. It doesn't has any UI (user interface). The service runs in the background indefinitely even if application is destroyed.

Generally, in android to create a service we must create a subclass of Service or use one of existing subclass. In android the application component such as an activity can start the service by calling startService() which results in calling the service’s onStartCommand() method.

Types of Services in Android When to Use: When a task needs to run in the background while the user is aware of it. Example: Music player, file upload, GPS tracking. These services require a visible notification and higher priority execution. When to Use: When a task needs to run in the background without user awareness.

The Android platform provides and runs predefined system services and every Android application can use them, given the right permissions. These system services are usually exposed via a specific Manager class. Access to them can be gained via the getSystemService() method. The Context class defines several constants for accessing these services.