WordPress a free Content Management System (CMS). The most popular CMS at the moment. WordPress started out as a blogging platform but grew out to a full website and webshop system. Currently WordPress is being used in more than 60 million websites.
The CMS works with themes, templates and plugins. The theme being the basis of your website, the theme contains templates for your pages and posts. Plugins can be added to your theme to expand it with features or improve performance. With more than 60 million websites comes a huge community. Large markets for both themes and plugins have been built, there is a theme or a plugin for almost everything. Complete businesses have been created revolving around both of these.
Using WordPress and it being open source, also offers the opportunity to use your own custom child theme. This way you can set up your own templates for your pages, posts, categories and so on. You can have complete control over your theme. Of course this demands a certain amount of knowledge of HTML, CSS and PHP, but nothing that can’t be learned.
WordPress also has very extended documentation including a complete custom theme handbook. When you can’t find what you are looking for over there, you probably will on different forums like Stack Overflow.