A theme is a collection of templates and stylesheets used to define the appearance and frontend display of a website powered by WordPress.
They can be changed, edit, and added from the WordPress admin area or call backend under Appearance » Themes. There are thousands of free WordPress themes available and thousands of paid available too. WordPress.org has a big collection of free ones in their themes directory. The advantages of paid themes are like those of paid plugins. paid themes usually have a reliable code base and offer support if something goes wrong. Paid themes are more powerful than free themes. Good themes improve standards with your website’s content in addition to being beautiful.
Each one comes with a different design, layout, and features it depends on what purpose that is created. 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 kinds of websites, e.g. WordPress corporate themes are designed for corporate company’s websites.
Overall WordPress themes are the templates that make your website look the way it does.