A sense of belonging
Having a community website, or extending an existing website to give it community features, is one of the best ways to encourage visitors
to your site. The visitors of a community website generate content for you, allowing you to build up a library of useful information for potential visitors.
More information about community websites in general
In a community website, discussions take place around your chosen topic through a variety of means, some technical, some social. A typical community website will feature at least some type of online forum, and possibly a chat room, blog, user-contributed photo gallery, FAQ system, and other such social software.
More information about online forums
Many successful community websites started off as blogs, which gained popularity over time. When you're getting dozens or even hundreds of comments posted to a given article, it makes sense to open up a forum, and then possibly a chatroom - and when your users have made a bunch of useful and informative posts which you'd like to preserve and promote, you might think about arranging them in an FAQ system. Perhaps you'd like some games in there too.
Such scenarios often take place. Sometimes the owner will find themselves with a thriving community without even really trying.
A community website can be something as simple as a forum based around a particular topic - or it can be a massive website with gigabytes of information and several distinct ways for members to talk to each other.