WordPress is an open-source content management system (CMS) that can be used for anything from a personal blog to a full business site.
The program is based around PHP and MySQL, but you don't have to a web development expert to get everything to work. Support for templates and themes mean you're able to customise the look and feel of your site quickly, without complex coding.
An extensive feature list includes full blog post comment support (with Trackback and Pingback), with robust spam protection; a full registration system so you can optionally have people create profiles before they can comment; there's an option for password-protected blog posts that will be hidden from the public; and there's easy importing from other blog platforms, including Movable Type, Textpattern, Greymatter, Blogger, and b2.
Need even more functionality? Take a look at the plugins - there are thousands freely available, many of which are amazingly powerful. BuddyPress is a perfect example. Install this any you can almost immediately create a custom social networking site of your own. Your users are able to register, create profiles, post personal messages, add friends, create and come together in groups, post directly to their Facebook and Twitter streams, and more. It's hugely comprehensive, and can be extended further with plugins of its own.
WordPress 3 is a major upgrade and ships with a lighter control panel user-interface, 1,217 bug fixes and feature enhancements, plus bulk updates so you can update all your plugins in one go.