Android Debug Bridge (adb)

From: Android Developers site

Android Debug Bridge (adb) is a versatile tool lets you manage the state of an emulator instance or Android-powered device. It is a client-server program that includes three components:

   * A client, which runs on your development machine. You can invoke a client from a shell by issuing an adb command. Other Android tools such as the ADT plugin and DDMS also create adb clients.
   * A server, which runs as a background process on your 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.

You can find the adb tool in <sdk>/platform-tools/.

When you start an adb client, the client first checks whether there is an adb server process already running. If there isn't, it starts the server process. When the server starts, it binds to local TCP port 5037 and listens for commands sent from adb clients—all adb clients use port 5037 to communicate with the adb server.