A WordPress theme is a group of files (graphics, style sheets, and code) that dictates the overall appearance of your blog or website. Themes can control something as vast as your site’s layout, or as minute as your hyperlink colors. It’s easy to think of a theme as a ‘skin’ for your site, but they are much more powerful than that.
A WordPress theme changes the design of your website, often including its layout. Changing your theme changes how your site looks on the front-end, i.e. what a visitor sees when they browse to your site on the web. There are thousands of free WordPress themes in the WordPress.org Theme Directory, though many WordPress sites use custom themes.