Joomla Multi Sites allows multiple domain names or sites to have their own joomla configuration file using the same joomla installation.
With this new feature you can share the joomla files and folders create web sites.
This simply the maintenance of Joomla as you have only one instance to update for all the websites.
With Joomla Multi Sites you can both create website from the back-end and from the front-end. You can use website as template to create new slave site. When you decide to propose the creation of website from the front-end, you can add a plugin to create a bridge with your own system to process the payment or whatever else.
As each website has its own joomla configuration.
You can decide to use:
- One database for all the websites. In this case, you must specify different database table prefix for each websites.
- Two databases (One master database and One slave database). This architecture is advised when you want to install Joomla Multi Site on an existing website and you are limited in the number of databases you can create (or disk space).
- As many databases than there are websites. This is the most flexible architecture where each websites works with its own database. This is generally the architecture adopted by joomla when you ceate multiple websites.
The content of each website is totally independent. This means that the content (articles, news, forum, users, ...) are NOT shared.

The setup of each configuration site consists in using the standard joomla installation mechanism.
In the administration of a 'Slave' site, you can re-install/uninstall extension previously installed on the 'Master' site.
The 'Master' site is the one where you have installed the Joomla MultiSites component and where you manage the 'Slave' sites.
Additional control are performed during the installation/uninstallation of an extension in a 'slave' site.
Before executing the standard joomla installation, it is verified that the extension you request to install has exactly the same name and same version number to avoid inconsistency between all sites.
This additional control is performed to ensure that you will overwrite the current installation with identical files that you originaly installed.
On 'slave' site, the uninstall of an extension consists in a joomla standard cleanup of the database. The files present on disk for this extension are not deleted to avoid disturb the other sites.

With the new version 1.1, you can also replicate a website into the same database to avoid re-installing the extension in each slave site.
For Unix platform, it is also possible to deploy a slave site into a specific folder.

Templates and Languages are automatically shared between all sites.
The re-installation of thoses extension is therefore disabled from the 'Slave' sites.
It is possible to customize the some folders (images, media, templates)

Joomla Multi Site is a Joomla native 1.5 component.