This module simplifies the process of converting single site to multisite.
Multisites is a capability of Drupal to run many web sites out of one installation of Drupal.
With multisite setup, you can have different domains and subdomains and paths mapped to different directories within /sites.
So, for instance, while Drupal multisites can support two different websites like example.com and foo.example.com, it can also support different settings and databases for example.com and foo.example.com.
For Drupal 7 multisite setup following steps are required:
1. Install Drupal 7 the normal way on the first domain (example.com).
2. Make your DNS work for the second site's domain (foo.example.com) to point to same IP.
3. Make your Web server work for the second site's domain to point to same directory.
(for apache this can be adding the second domain as an alias in the current host or making a new virtual host that has the same document root)
4. Make a new directory with name equal to the second site's domain in under the sites directory (drupal7/sites/foo.example.com)
5. Copy default.settings.php to the second site's directory as settings.php (drupal7/sites/foo.example.com/settings.php) and then chmod it to 666