WordPress Network was previously installed using WordPress Mu, which has since been discontinued. However since WordPress version 3, you can have more-or-less the same functionality using WordPress.
A multisite network is your own version of WordPress.com where your users can create their own blog / sites on demand. You can restrict it so only you, the administrator, can create new sites.
A multisite network is a collection of sites that all share the same WordPress installation. They can also share plugins and themes. The individual sites in the network are virtual sites in the sense that they do not have their own directories on your server, although they do have separate directories for media uploads within the shared installation, and they do have separate tables in the database.
The detailed instructions can be found in WordPress Multi User.