Wordpress is one of the most popular Content Management Systems on the web at present. It's free and has been built up from a blogging platform to now having added functionality so that many big-name websites are running on wordpress.
Why is it so popular? - it's easy to get going without any web design skills - just point and click with some pre-defined themes. In addition developers have been busy adding plugins that do all the other things you may need (ecommerce, calendars, quizzes etc.)
It's more user friendly than Drupal (but some argue not as robust) and has a huge online community that can solve common WP problems. If you don't have design skills then there are a whole heap of themes out there that you can pimp and tweak to get a fast, good looking site.
Check out this video that shows a quick introduction into the world of wordpress.
If you're interested check out the official site:
Personally I'm not a big fan of wordpress, but I can see why it's so popular - the same reason that the web devil blog is banged out on blogger rather than a hand-crafted website.
Conclusion: A solid CMS platform with lots of support and thousands of plugins to extend the basic functionality. Great to get a site up and running with a predefined theme - more of a learning curve to make your own theme or a website layout in your own style.
What are the Pros and Cons of your Wordpress setup?