Whats a WordPress Theme?

A WordPress Theme is a set of files that interact to generate the look of your site.

For instance this site uses the HeatMapTheme were as over on MyWordPress the Stallion SEO Theme is implemented.

These WordPress Themes again are plug and play so require little more then uploading to your site and activating.

There are thousands of WordPress themes to choose from ,take a look here , and if you cannot find what you are looking for you can design your own or buy one of the many commercial WordPress Themes that are available.