In website development, CMS stands for Content Management System. It is a type of website that allows logged-in users to update and change information and images. To update content, like text and photos, the logged-in user uses a visual/text editor, similar to a Microsoft WordPad but with less toolbar options. (See figure 1). A logged-in user who wants to change information on the site does not need to have prior knowledge of HTML web markup, although a little bit of HTML knowhow couldn't hurt either.
There are a plethora of CMS's today. They can be grouped according to programming languages. Java has dotCMS, Microsoft ASP.net has C1, Perl has Foswiki and Python has Django. PHP, the server-side scripting language of the web, has concrete5, Drupal, Joomla!, Magento and of course Wordpress to name a few. Wikipedia has an exhaustive list of content management systems grouped according to programming languages.
A lot of businesses choose Wordpress as their CMS platform. The reason for this is maintainability, since there are more web developers who are knowledgeable in Wordpress. You don't want to get stuck with some exotic CMS that only a handful of web developers know how to fix or maintain.
Drupal is also a popular CMS among big enterprise and government websites. Drupal emphasis on security makes it a great candidate for security conscious websites. But the caveat is that one would need a dedicated website management team to monitor and manage the back-end.
Web Design 8 has been building modern PHP based CMS websites like Wordpress and Drupal professionally since 2015.