Here is the definition from the www.wordpress.org website: WordPress is a state-of-the-art publishing platform with a focus on aesthetics, web standards, and usability. WordPress websites and blogsites can be completely custom built from scratch, or an existing WordPress theme or template can be used as a based and then customized. Basically, WordPress is what you use when you want to work with your blogging software, not fight it.
WordPress is an Open Source project. That means hundreds of people all over the world are working on it and creating applications and plug-ins that work with it. It also means you can use WordPress for any website you want without being required to pay anyone a license fee. It was originally designed just for blogging. But because it is so powerful, many developers have created functional and helpful plug-ins that work with WordPress, making it a great tool for the development of business websites as well.
In a standard HTML/CSS website, each page and all of its content is a separate file and in most cases, the page files contain the code for the design and layout as well as the content. Anytime you want to make a change, you have to make it to each page it affects and re-upload the updated files to your server through an FTP account.
With WordPress, all of the content for each page is stored in a single database. The design and aesthetics of the site are stored in separate files that define the theme (template/skin) of the blog site. This makes updating the design and layout of the site much easier, because you only have to make revisions to the theme files, not the content. Likewise, it means all of your content changes can be made in one place.