How to extend default user registration page in Drupal 7? I would like to extend the default user registration page by adding more fields that will be stored in the same users table of database with the default fields.

What is the easiest way to do that? Could explain how to achieve this?
In Drupal 7, you just add the fields to the user entity in "Administration » Configuration » People » Account settings » Manage fields."
Then, for every field there is the "Display on user registration form" checkbox.