Joomla Multi Sites allows multiple domain names or sites to have their own joomla configuration file using the same joomla 1.5.x installation.
In other words, Joomla Multi Sites share the Joomla files and folders present on the disk (PHP code) and use different database connection to access the website specific content (data).
In very brief, Joomla Multisite share the PHP code but use specific data that make each website totally independent(*).

In summary Joomla multiple sites allows:

* Create fresh slave site
* Replicate existing website into the same DB or into another DB
* Possibility to propose a service for the website creation from the front-end
* Possibiillty to share the users (**) – Single Sign-in:
o The standard joomla users
o The VirtueMart users
o All the community builder
o All JoomSocial
* Possibility to share a limited number of extensions (**)
o All VirtueMart
o All SOBI2
o …. (The exhaustive list is present in the feature section).
* Synchronise the install/sharing extension with its “parent” website (the website that is defined in a replication rule)
* Tool to supervise the websites dependencies and also possibility to distribute the install/sharing/uninstall a limited number of extensions into of a website and its dependencies.
* Reduce the maintenance cost as the PHP code is installed only once.
The bug fix and security fix require can be installed only on the master to have all websites automatically benefit of the update.
* Image, media and template directories may be specific.
By default they are shared by all the websites.
* Possibilty to deploy the websites into a specific directory using Symbolic Links (**) .
This allow for example let your cPanel, Plesk, …, hosting administration tool create the directory associated to a domain or sub-domain and ask later to Joomla Multi Sites deploy the website into this specific directory.
* Joomla multisite is a native Joomla 1.5.x component.

Remarks:
(*)The websites are independent in terms of the content.
The extension present in a slave site must be prevously installed (present) on the master website.
Only the themes (template) may be specific and not present on the master website when the slave site use a specific directory for thoses template. See user manual for more detail.
(**) User sharing and extension sharing require MySQL 5.0 or higher.
This also depending on the DB login permissions. Some hosting provider may require to use the same DB to share the joomla users and extensions.
(***) Symbolic Link is a feature only available on unix platform.
Some hosting provider may disable this facility.