An Android activity is a single, focused thing that a user can do. An activity represents a screen in an app that the user can interact with. An activity is responsible for creating a UI, handling user input, and processing results.

A fragment, on the other hand, is a portion of the user interface in an activity. Fragments are used to organize UI components and handle different screen configurations. With fragments, you can reuse UI components in multiple activities, and you can create dynamic and flexible user interfaces.

The key difference between activities and fragments is that activities are independent and self-contained, while fragments are designed to be combined with other fragments within an activity. Fragments are often used to represent a portion of the UI in a tablet app, while activities represent the entire UI of a phone app.

In summary, activities are the entry points to an app and define the navigation between the different screens, while fragments are the UI components that can be combined to create a complete screen within an activity.