In WordPress, a theme is a collection of templates and stylesheets used to define the appearance and display of a WordPress powered website.
They 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.
Each one comes with a different design, layout, and features. A user needs to choose the one that suits their taste and requirements for their website. There are those that are designed to serve specific kind of websites, e.g. WordPress Photography Themes are designed for photographers and photography websites.