A Handler allows you to send and process Android.OS.Message and Runnable objects associated with a thread's Android.OS.MessageQueue. Each Handler instance is associated with a single thread and that thread's message queue. When you create a new Handler, it is bound to the thread / message queue of the thread that is creating it -- from that point on, it will deliver messages and runnables to that message queue and execute them as they come out of the message queue.

There are two main uses for a Handler: (1) to schedule messages and runnables to be executed as some point in the future; and (2) to enqueue an action to be performed on a different thread than your own.

Scheduling messages is accomplished with the Handler.Post(Java.Lang.IRunnable), Handler.PostAtTime(Java.Lang.IRunnable, System.Int64), Handler.PostDelayed(Java.Lang.IRunnable, System.Int64), Handler.SendEmptyMessage(int), Handler.SendMessage(Message), Handler.SendMessageAtTime(Message, System.Int64), and Handler.SendMessageDelayed(Message, System.Int64) methods. The post versions allow you to enqueue Runnable objects to be called by the message queue when they are received; the sendMessage versions allow you to enqueue a Android.OS.Message object containing a bundle of data that will be processed by the Handler's Handler.HandleMessage(Message) method (requiring that you implement a subclass of Handler).

When posting or sending to a Handler, you can either allow the item to be processed as soon as the message queue is ready to do so, or specify a delay before it gets processed or absolute time for it to be processed. The latter two allow you to implement timeouts, ticks, and other timing-based behavior.

When a process is created for your application, its main thread is dedicated to running a message queue that takes care of managing the top-level application objects (activities, broadcast receivers, etc) and any windows they create. You can create your own threads, and communicate back with the main application thread through a Handler. This is done by calling the same post or sendMessage methods as before, but from your new thread. The given Runnable or Message will then be scheduled in the Handler's message queue and processed when appropriate.

[Android Documentation]