Dalvik Debug Monitoring System:

DDMS is integrated into Eclipse and is also shipped in the tools/ directory of the SDK. DDMS works with both the emulator and a connected device. If both are connected and running simultaneously, DDMS defaults to the emulator.

• From Eclipse: Click Window > Open Perspective > Other... > DDMS.
• From the command line: Type ddms (or ./ddms on Mac/Linux) from thetools/ directory

  How DDMS Interacts with a Debugger

  When DDMS starts, it connects to adb. When a device is connected, a VM monitoring service is created between adb and DDMS, which notifies DDMS when a VM on the device is started or terminated. Once a VM is running, DDMS retrieves the the VM's process ID (pid), via adb, and opens a connection to the VM's debugger, through the adb daemon (adbd) on the device. DDMS can now talk to the VM using a custom wire protocol
  
  
  DDMS features summary:/when we will use DDMS ?
  For the following purpose, we can use DDMS.
  1.Viewing heap usage for a process
  2.Tracking memory allocation of objects
  3.Working with emulator or device's file system 
      we can copy video files/files to emulator or device with DDMS
  4.using logcat : logs can be viewed and filtered based on LOG_TAGS
  5.monitoring network traffic information
  
  
  
  
  
  
  
  
  
  
  
  
  Source From:http://developer.android.com/tools/debugging/ddms.html