Android Core Concepts

• Apps and APK Files
• Activities
• Fragments
• Views and ViewGroups
• Layout XML Files
• Intents
• Widgets
• Services
• Sensors

Activities

An Android activity is a GUI component. It corresponds roughly to a window in a desktop application. Since mobile phone screens are small, an activity typically takes up the whole screen. If you open multiple activities they are stacked on top of each other. You cannot arrange activities side by side, like you can do with desktop windows.

Fragments

In Android a fragment is a fragment of a total user interface. A fragment typically only takes up part of the screen. Fragments are used inside activities. Fragments can be reused within different activities.

Layout XML Files

Activities, fragments and some ViewGroups can use XML files to define their layout and contents. The layout XML files specify what GUI components an activity or fragment contains, as well as styling of the GUI components (size, margins, padding etc.).

Intents

Android intents are small objects that an activity can pass to the Android operating system, to tell the operating system that some other action or actvity is required. 

Widgets

Android widgets are GUI components which can be displayed outside of an activity. 

Services

Android services are background processes that can be executed on an Android device, even if no application is visible.

Sensors

Android devices typically have a set of sensors built in which you can access from your Android applications.