Custom Module

I have read so may article on 'How to make custom module'. There are lots of bloggers who write at at-least once about "How to make a module or custom module" that's why I have decided to write about module and custom modules. The module is used to extend the functionality of Drupal and there are more than 25000+ modules freely available for use. Different-2 modules are used for different-2 purposes like for e-commerce Drupal-commerce or Ubercart, for social business solution Drupal-Commons, for page view View, etc.