WordPress’ own site states pretty effectively what the project is, where it came from, and who it’s for:
WordPress started as just a blogging system, but has evolved to be used as full content management system and so much more through the thousands of plugins, widgets, and themes, WordPress is limited only by your imagination. (And tech chops.)
Started in 2003, the WordPress project was designed to bring blogging easily to the masses. Over the years, they’ve expanded far beyond their original scope, and they now run one of the largest hosted blogging solutions out there. Founded by Matt Mullenweg and supported by Automattic, WordPress offers two distinctly different solutions: hosted sites (via wordpress.com) and the open-source platform files (via wordpress.org).
I sincerely think during WordPress’ evolution over the years, it has woven itself into the fabric of the Internet. For the first time, a single platform allowed both grandmas and Web developers the same tools to create online sounding boards for discussion, rants, raves, reviews, or just plain out-loud thoughts. Not only do they do this well, but the two offerings allow as little or as much customization as the user wants. This ability to customize, paired with how simple it is to get a WordPress blog up and running, helped shape the Internet as we know it today.