WordPress is a comprehensive, state-of-the-art, blog publishing platform and content management system with a focus on aesthetics, web standards, and usability. It is the largest self-hosted blogging tool in the world, used on millions of sites and seen by tens of millions of people every day.
WordPress is W3C compliant and an Open Source project, which is a development method for software that harnesses the power of distributed peer review and transparency of process. The promise of open source is better quality, higher reliability, more flexibility, lower cost, and an end to predatory vendor lock-in. That means you are free to use it for almost anything without paying anyone a license fee.
There is also a service called WordPress.com which lets you get started with a new and free WordPress-based blog in seconds, but varies in several ways and is less flexible than the WordPress you download and install yourself.