A service is an Android component that runs in the
background to perform long-running operations or to
perform work for remote processes and does not provide a
user interface. An activity can connect or bind a service that
is running. (if the service is not running, launch it). When
connected to a service, the activity can communicate with the
service through the interface that the service exposed. Like
other application components, service components always
running in the main thread of an application by default. So
for the intensive or blocking operating a service performs
(may slow down activity performance), it is usually start a
new thread inside the service.