You may have heard people refer to a Content Management System (CMS) - especially in the context of building a website. So what is a CMS?
Going back many years, websites used to just be a series of flat HTML files. If the website had 5 pages, there were 5 HTML files which explictly outlined the content of those 5 pages in their entirety. If you wanted to create a website, you either needed to be able to understand HTML, or you needed to use a program like Dreamweaver or Frontpage to help.
However, the technology powering websites then started to improve. The content (words, images, audio + video) could be stored separately to the layout. This meant that one could entirely change the look of a website while keeping the content intact (and vice versa). And thus, CMSes were born.
These days, almost every website you visit is built on top of a CMS. At the moment, most are built on top of one of the big three CMS systems - Wordpress, Joomla and Drupal. However, even simpler systems like Squarespace are increasing in popularity. This is likely to eat into Wordpress's market, as Wordpress is currently the go-to for the most basic websites. The rise of Squarespace is unlikely to affect Joomla and Drupal as much, as these CMS tend to be chosen to run sites with features more advanced than Squarespace can provide.
Whichever CMS you choose, the basic concepts are the same. You're able to log into an admin panel for your website. From there, you are given an easy-to-use graphical interface that allows you to edit the content on your website. You're able to do this without having to worry about the layout of the site.
Layout is dealt with separately. You can either use one of the templates provided with the CMS, install a 3rd party template, or have a bespoke template built. Regardless of your choice here, these templates are applied sitewide, and include areas for your content to appear.
Many of the popular CMSes also have a thriving plugin community. This enables you to extend the functionality of the base CMS to include all sorts of extra features. These include forms, sliders and shops. Almost all of these can be implemented without getting near any form of website code. However, with great power comes great responsibility. As these plugins can vary in quality, it's possible to break your website or leave easy avenues for hackers to get into your website. Platforms like Squarespace avoid this by preventing users from touching the website code in any way, therefore preventing anyone (other than Squarespace themselves) from extending its features.