Wordpress is a basic content management system or CMS. The content that is managed is the information that you want to display on your website. Here is Wordpress' official definition:

"WordPress is a state-of-the-art publishing platform with a focus on aesthetics, web standards, and usability. WordPress is both free and priceless at the same time."

What does all that mean? Well, Wordpress is nothing but an application, like Microsoft Word or Entourage, that helps you manage stuff, information, pictures or whatever. It does this with a database that stores all this "stuff" and then instructed by you on how to display it to your customers.