WordPress themes are the collection of templates and style-sheets used to define the appearance and end-user display of a website.
"WP Theme can be changed, managed, and added from the WordPress admin area under Appearance » Themes. There are several free and paid WordPress themes available. WordPress.org has a large collection of free ones in their Themes Directory."
Endless Collection & Still Growing
Each WordPress theme comes with a different design, layout, and features. You can choose the one that suits your taste and requirements of the website. Choose according to Features, and Layout for various purpose.