This article will be explaining how to create custom block and how to add the custom block in to quick tabs, programmatically. If you want to display the block inside a node you can refer our artilce How to print a block inside a node on a Drupal 6 website.
First lets create a custom block using hook_block. In this block what I want to display is when we provide a url, need to display corresponding alexa graph for the particular site.