A content management system (CMS) is designed to make website creation easy! The cool thing about a CMS is that it allows you to edit your website with only your browser! It is essentially a bit of software that is uploaded onto your server. Now when you access your website, it allows you to add pages, modify styles, and more users that can modify your site and so on and so on. No need for a HTML/PHP/CSS editor on your computer and a ftp uploader. It also means you can edit your website from anywhere in the world, from the lowest of powered netbooks and smart phones. They also make website creation easy for non-programmers with WYSIWYG editors (what-you-see-is-what-you-get, kinda like Microsoft Word), add-on modules which can huge amounts of functionality to your website, and heaps of pre-built, one-click designs to make your site sparkle.
Information about the Wordpress platform.