When to use handler.post() & when to new Thread()

I'm wondering when should I use handler.post(runnable); and when should I use new Thread(runnable).start();

It is mentioned in developers documentation for Handler:

Causes the Runnable r to be added to the message queue. The runnable will be run on the thread to which this handler is attached.

Does this mean if I write in the onCreate() of Activity class:

Handler handler = new Handler();
handler.post(runnable);

then runnable will be called in a separate thread or in the Activity's thread?

You should use Handler.post() whenever you want to do operations on the UI thread.

So let's say you want to change a TextView's text in the callback. Because the callback is not running on the UI thread, you should use Handler.post().

In Android, as in many other UI frameworks, UI elements (widgets) can be only modified from UI thread.

Also note that the terms "UI thread" and "main thread" are often used interchangeably.

Edit: an example of the long-running task:

mHandler = new Handler();

new Thread(new Runnable() {
  @Override
  public void run () {
    // Perform long-running task here
    // (like audio buffering).
    // You may want to update a progress
    // bar every second, so use a handler:
    mHandler.post(new Runnable() {
     @Override
     public void run () {
       // make operation on the UI - for example
       // on a progress bar.
     }
    });
  }
}).start();

Of course, if the task you want to perform is really long and there is a risk that user might switch to some another app in the meantime, you should consider using a Service.