A theme is a collection of files that alter the visual aspect of an application, program or website.
In WordPress, a theme is a set of files that provide a visual style and customised functionality to a WordPress site. These file can include template, function and other code files (written in PHP), style-sheets (using CSS), and images files (often JPG and PNG files).
A theme often interacts with other WordPress software like plugins which can increase or reduce the available features of a theme.
Themes can be free or paid, most themes offer both with the free version providing fewer options that the paid version.
Themes can be manually downloaded from wordpress.org or installed directed into a WordPress site from the site’s admin area.