http://en.wikipedia.org/wiki/WordPress
Alternatives to Wordpress:
Wordpress itself is split into two, since you mentioned it. There's Wordpress.org, where you can download the splendid open source blogging tool that's preferred by many of my colleagues, and there's Wordpress.com, which is their free hosted blog service. I used to be fairly enthused about the latter, but since it's come to light that they enforce their no-commercial-content terms of service in a rather scattershot manner (see Is Wordpress the Blog Police? for an explanation), I no longer recommend that you use this service for your blog, nor any similar service. After all, how painful would it be for you to try and log in to your blog just to find it's "Shut down due to violations of our service terms. Good bye."
But not to throw out the baby with the bathwater, Wordpress the downloadable blogging software package is really a strong contender for any blogging effort, whether you want to become a movie reviewer or whether you're a Fortune-50 company ready to take the plunge.
Nonetheless, what I recommend are the blogging tools from SixApart.com, notably TypePad, their not-free-but-darn-inexpensive hosted blog service, and Movable Type, their commercial grade blog package that I use here for AskDaveTaylor, among many other sites.
I hope that helps you find clarity between these different tools and start out with the platform and service that best matches your own needs and goals in the blogosphere.
