Multisite is a special "mode" that is now implemented into WordPress that allows for multiple websites to be run under one installation of the CMS. This also includes the installation of Themes and Plugins to a top-level user, making them available to all of the sub-sites below it. This is great if you have a website and you wish to host multiple blogs that have an associative purpose, whilst allowing each blog to appear as a unique website. You'll be able to set a super-admin, which will allow you to administrate all websites associated from the one admin area. You can then set up sub-admins, for specific access to each of the additional websites whilst retaining use to the same pre-installed resources.
So how do you implement this mode to your current WordPress website?
Firstly, you should prepare your Wordpress settings. Updating your network and settings will potentially change your website. Unless this is a fresh install and you have nothing to lose, please back-up your website. Also deactivate all active plugins. You can reactivate them again after the network is created.
Next, you can go ahead and allow multisite. Within the public_html for your WordPress installation, there's a file named wp-config. Open this file up and add the following line;
/* Multisite */
define( 'WP_ALLOW_MULTISITE', true );
Then save the file and refresh your browser.
You're now ready to set up a network. You'll now see "network setup" under the tools menu of your Wordpress admin area, from here, select "create a network of wordpress sites." You are given the choice between sub-domains and sub-directories, except when existing settings restrict your choice. PLease think about the best long term choice, as once it's set you will not be able to change this. It's worth noting that for a subdomain setup, you will need a wildcard A record for all subdomains of the website to point to the server IP, this can be done via the DNS editor in your control panel. You'll also see a section for "network settings", these should be filled in automatically, but please ensure the details are correct. When you're done, Click install. This will perform and automatic check on the network settings - if anything does need editing, the installer will let you know.
The next step is to enable the network. Once the install has initiated, the next thing you'll be presented with is some specific lines of code to add to both your wp-config file and your .htaccess. The location of which is clearly marked by the installer. If you ALREADY have a .htaccess file, replace any existing WP lines with these new ones. In some cases you might also have to add "Options FollowSymlinks" at the start of the file. After completing these steps, log in again using the link provided. You might have to clear your browser's cache and cookies in order to log in.
You can now set up your network admin preferences. The "My Sites" option will now be available from the top of the admin screen. This contains a network admin table, you can now use this to configure the network as you add and remove new sites.