What is Multisite?
I was recently asked if multisite would be a good solution for someone who wants to separate access to the administration of a website from access to the administration of a plugin? The WP user was already using the Member and Adminize plugins to achieve this but thought they were overkill and that multisite might be a simpler solution.
My answer to this was that multisite would certainly be even more overkill and that it is not really what multisite is designed to do.
Multisite basically allows you to create a network of websites, similar to WordPress.com. They are separate websites with their own databases but they run off the same WordPress installation and they share plugins and themes. A Super Admin has privileges to administrate the whole network while Admins can only administrate their own subdirectory blog. The Super Admin decides which themes and plugins will be available for use within the network. The admin of each blog decides which one of those plugins they want to use.
It is an efficient way to administrate many websites at once. It is also a good way to preserve branding over a number of websites while at the same time allowing each website independent publication privileges.
Here is the informative presentation Boone Gorges gave at the May 2010 WordPress Meetup.