Similar to wordpress, Drupal allows contributors to create modules to enhance its feature. Just like those plugins in wordpress.
To install the modules, download the module package. I use the ImageAPI module as an example.
1. Download the ImageAPI module – Download Page
2. Create a folder /var/www/drupal/sites/all/modules
3. Extract the imageapi-6.x-1.6.tar.gz and move it to the above modules folder
4. You can now find the ImageAPI module on the Drupal administration panel
Done =)
Reference:
Eureka! 