In the past, there used to be a separate version of WordPress named WordPress MU (pronounced as myoo) that allowed you to create a master blog with many sub-blogs—essentially giving each user their own (limited) blog.
Well, as of WordPress 3.0, this capability is a built-in feature in WordPress, and just needs to be enabled. If you go to the old WordPress MU URL (http://mu.wordpress.org/), you'll see the following:
You may be wondering what makes a WordPress site, which can have multiple authors, different from a WordPress MU site. Here's the official definition from wordpress.org:
A multisite network is a collection ...