WordPress is an open source blog engine. Open source means that nobody owns it, everybody works on it, and anyone can contribute to it. Blog engine means a software that can run a blog. It's a piece of software that lives on the web server and makes it easy for you to add and edit posts, themes, comments, and all of your other content. More expansively, WordPress can be called a publishing platform because it is by no means restricted to blogging.
WordPress was originally a fork of an older piece of software called b2/cafelog. WordPress was developed by Matt Mullenweg and Mike Little, but is now maintained and developed by a team of developers that includes Mullenweg.