The WordPress Codex says it best, “WordPress Themes are files and styles that work together to create a presentation or look for a WordPress site.
Each Theme may be different, offering many choices for users to take advantage of in order to instantly change their website look.”
There are thousands of free WordPress themes readily available. You or a skilled WordPress developer can create your own custom theme if you prefer. If you decide to build your own WordPress Theme there are several good books available. You can begin at the WordPress Codex…