WordPress is a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usabiliy. What a mouthful. WordPress is both free and priceless at the same time.
More simply, WordPress is what you use when you want to work with your blogging software, not fight it.
WordPress started as just a blogging system, but has evolved to be used as full content management system and so much more through the thouands of plugins, widgets, and themes now available. WordPress is limited only by your imagination. (And tech chops.)