We could write an entire essay about this one, but multisite is a different way of using WordPress.
A multisite allows you to run and manage lots of different WordPress sites from one install. It makes it a lot easier to keep all of your websites updated in one place.
You can also manage who has access to which sites. For example, you might have access to the whole network, but then create users for specific sites so that person can only access that site, rather than all of them.
There is a new user role within a multisite, called a ‘Super Admin’. This means they have access to all of the sites across the install and can access the settings for the main install also.
The media uploaded within a multisite also works differently, as any media uploaded to an individual site will only be on that said site. You can’t access the images from one site to another.
Multisite installs are a great way for larger companies to create separate websites for different departments, for example, whilst being able to access and manage them all in one place.
Another great way to use a multisite is if you wanted to run a blogging platform but were trying to keep topics separate or away from your main site. You could set up a multisite install and keep everything managed and easily updated within one place!