An Activity is a single focused thing that the user can do.
Typically an Activity will correspond to a screen in an application. but keep in mind that an Activity can also
be a floating window on top of another Activity.
An application is typically composed of a set of components, including activities, services, and broadcast
receivers, that are run using the same user ID and process on the platform. As users click on buttons or
respond to notifications to open new activies, the system shuffles existing activities to the background.
lifecycle methods:
onCreate: called when an Activity is first created, is where things are initiated.
onPause: called when an Activity is going into the background, is where they should be cleaned up
or persisted.
onResume: called when an Activity is being resumed after having been in the background. is where
things are reloaded or reset.
In addition to the lifecycle phase hooks, Activity also extends Context and provides a host of event,
state, menu, and other helper methods.