Online Android SDK Manager

The Online Android SDK Manager is a online tool that allows you to Download and update packages for the Android SDK.
The sdkmanager tool is provided in the Android SDK Tools package (25.2.3 and higher) and is located in android_sdk/tools/bin/.
Once downloaded, you can extract the downloaded zip file in the appropriate directory indicated inside as the Android SDK Location.

SDK Platform

At least one platform is required in your environment so you're able to compile your application. In order to provide the best user experience on the latest devices, use the latest platform version as your build target. You'll still be able to run your app on older versions, but you must build against the latest version in order to use new features when running on devices with the latest version of Android.

Sources

Sources for Android Platforms.

Tools

Tools are useful for building, managing and controlling Android apps.

System Images

The system image is required in order to run the Android Emulator. Each platform version contains the supported system images.

Extras

Android Support Repository, Google APIs, Glass Development Kit Preview etc.