An application is a piece of software that brings additional features into the basic functionality. OnApp allows you to deploy a wide range of applications by means of additional software. To install different applications on your cloud, you shoud create an Application Server. An application server is a regular VS based on CentOS but with pre-installed software. Application Servers allow you to install various applications (like Drupal, Joomla, Wordpress etc.) on a server using web interface.