Activity is the presenter of a single screen in the application. It is the only one who has certain abilities, like displaying Views, menus, alerts and notifications. It also has the ability to open a new Activity, which means opening a new screen.
Activity is a class which derives from android.app.Activity.
An application needs to have at least one Activity.
All Activities must be declared in the manifest file.
The Activity class defines the following call backs i.e. events. You don’t need to implement all the callbacks methods. However, it’s important that you understand each one and implement those that ensure your app behaves the way users expect.