With WordPress, you have the ability to create a network of sites (Multisite). This article is instructions for creating a network (multiple sites).

1. First of all if you are running not fresh wordpress system, make back up off all you data.
2. Open your config.php file and paste this above where it says /* That's all, stop editing! Happy blogging. */:

   <define('WP_ALLOW_MULTISITE', true);

   This allow to use multiple sites in your wordpress.

3. Now install a network. Go to Administration > Tools > Network. Enter all required data ant press install.
   • Back up your old wp-config.php and .htaccess files.
   • Create blogs.dir directory in folder wp-content/
   • Add extra lines to your wp-config.php and replace your .htaccess file rules with new once. These rules are dinamically generated on your network configuration page.
   • Now log in again to system.

More information about this you can find in http://codex.wordpress.org/Create_A_Network.