Now that you are done with Installing Drupal 7, you can add additional functionality to your site by Installing contributed modules, or you can customize the look and style of your site by Installing contributed themes.
The standard installation of Drupal core is both limited in functionality and is rather plain looking. Drupal has been designed, however, to be modular so that it is easily extensible with additional modules and themes.
Each of the multitudes of additional modules and themes available at Drupal.org is referred to as a ‘project’, and all of them are free of charge. They are often called ‘contrib’ because they were contributed by Drupal community members just like you.
In many cases, you will discover that someone has already written, and uploaded to Drupal.org, a project that provides exactly what you are looking for.
The process of finding a module or theme to use, and the process of importing it into your site, is essentially the same for both modules and themes.