Activities are short-lived and can be shut down at any time.
Services, on the other hand, are designed to keep running, if needed,
independent of any activity, akin to the notion of services or daemons
on other operating systems. You might use a service to check for
updates to an RSS feed or to play back music even if the controlling
activity is no longer operating. You will also use services for scheduled
tasks (“cron jobs”) and for exposing custom APIs to other applications
on the device, though those are relatively advanced capabilities.
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