ADB stands for Android Debug Bridge. It is a command-line tool that is used to communicate with the emulator instance. ADB can control our device over USB from a computer, copy files back and forth, install and uninstall apps, run shell commands, and more.
It is a client–server program that includes three components:
A client, which runs on our development machine. We can invoke a client from a shell by issuing an ADB command. Other Android tools such as DDMS also create ADB clients.
A server, which runs as a background process on our development machine. The server manages communication between the client and the ADB daemon running on an emulator or device.
A daemon, which runs as a background process on each emulator or device instance.