WordPress is a free and open-source content management system (CMS)
WordPress — freely distributed, standards-compliant, fast, light, and free content management system, with sensible default settings and features, and an extremely customizable core.
What Does WordPress Look Like
WordPress with default Twenty Seventeen theme
The closed part of the website, accessible to the administrator, where the configuration, management of the site and its content are carried out.
A WordPress Theme is a collection of files that work together to produce a graphical interface with an underlying unifying design for a weblog. These files are called template files. A Theme modifies the way the site is displayed, without modifying the underlying software.
There are free and commercial premium themes, with enhanced functionality and more customization options.
Sometimes themes are called WordPress templates — this isn’t right.
Plugins are ways to extend and add to the functionality that already exists in WordPress. WordPress Plugins are available from several sources.