Hello I’ve come from Joomla to ask if drupal do this?

Hi Roy.
Yes, you can do that.

You can write your own modules and, using hooks provided from Drupal core,
interact with the system and store additional informations that you need
in the database in your own tables.

A good starting place are the handbooks :

http://drupal.org/handbooks
http://drupal.org/contributors-guide
http://drupal.org/node/508

Roberto

It depends on how exactly you want the two systems to interact, but custom PHP can be used in several places in Drupal:

• Inside "blocks" (the stuff you see on the side here with "new forum posts" and the like) -- you can throw some custom PHP to show the 5 newest blog posts, for example.
• Inside "nodes" (any content you create; forum posts, pages, blog entries, etc.) -- you can throw some custom PHP to do everything from display the current date/time, to call in other pieces of content dynamically.
• Inside "modules" (these are actually all code; extensions which can make Drupal do fancier stuff) -- more advanced; you can do just about anything. ;)

For instance, each time a new user registers, you might want to save their user ID to a separate table for your external system. You could do this with a custom module that implemented a "hook_user" function:

// This would be in a file called 'custom_module.module' in the modules directory
function custom_module_user($op, &$edit, &$account, $category = NULL) {
  if ($op == 'insert') {
    // A new user just registered and is being added to the database.
    // Store that user ID value in the external system table.
    db_query('INSERT INTO external_system_table (user_id) VALUES (%d)', $user->uid);
  }
}

(note: that's totally untested, and it's very late here, but should be pretty close ;))

"Hooks" are Drupal's way of letting modules modify and extend "core" behaviour without hacking any core source code! They're very powerful and there are almost limitless things you can do with the flexibility.

Hope that helps answer your question.

Don't forget http://api.drupal.org

-----
http://para.ro