The engine of the website can be compared to a man. Engines for sites like people, there is a large number. Someone people can count, the other knows how to draw someone faster or smart, etc. and Similar engines for sites: one better suited to create a blog, another for an online store, and the third for the establishment of the forum, the fourth for torrent resources, etc. in Other words, the engine resource is its heart.
As you have seen, there are many types of CMS. There are free engines open source that applies a lot of users, they have many different themes, plugins, add-ons etc. There is a paid CMS with closed source, which are more suitable for commercial purposes and companies. Many experienced programmers write engines themselves, for their own sites. Today in Runet the most popular CMS: Joomla, 1C-Bitrix, WordPress, Drupal etc.
Some time ago, websites consisted of static pages. Ie if you have 100 articles posted, it meant that you have at least 100 different files (pages). The creation of these files, their content HTML code had to be done manually. In addition, these static files are stored on a server and had a large amount of disk space. In addition, the processing speed of static pages was low.
The site engine (CMS) is needed for the dynamic interaction between the website and the user, facilitate the work of the webmaster, the optimizer and the administrator.
The main advantage of the content management system of the site is its functionality in terms of making changes to the finished content. Suppose you want to add a new subscription form news or to change banner code. If the site is CMS that work you can do for a couple of minutes, no matter how many pages are on the site 100 or 1000. If you have no engine, this simple operation will cost great effort and will last for a couple of days.
To select a CMS, you need to decide what kind of content will you have on the website. If this will want an Internet store, it is best to use 1C-Bitrix, PHPShop, Simpla, etc. If you want to create an author blog or website-a business card, they can come WordPress, Joomla, etc.